Tack

As an object,
- A small sharp nail, such as one used to hook carpet in place.
- A rope used in sailing to change direction

As an action
- In sailing it means changing direction of the boat by tying the sail here and there, as needed.

Example
"I wished to reach you without loss of time, and I did what people will do in politics, or on the sea when the wind is against them, -- I tacked."
Johan de Witt to his brother Cornelius de Witt, in The Black Tulip by Dumas

Guerdon

Reward for an accomplishment or service.

John de Witt... too received a noble recompense for his devotedness to the best interests of his country, taking with him into the retirement of private life the hatred of a host of enemies, and the fresh scars of wounds inflicted by assassins, only too often the sole guerdon obtained by honest people, who are guilty of having worked for their country, and of having forgotten their own private interests.
from The Black Tulip, by Duma.

Mosey On

Walk slowly, as in leisure.

Example: There is one Calvin Hobbes strip with this word. Too good really.
Another, Lets mosey on home after dinner.

Boondocks

Rural place; more intense word than 'backwoods', or 'woods' or 'hinterland'.
The word comes from Philippino language, in which it means mountain. When American occupied the land towards the end of 19th century, they used this word to denote the mountainous hinterlands in Philippines.

Example
One of the few drawbacks to living out in the boondocks is that public services, such as litter collection, are less available than they are closer to the city.
Don't sweat the small stuff, by Richard Carlson

Au pair

A young foreigner who remains with a family as a surrogate family member, takes care of some of the house work such as baby-care etc. and in turn gets boarding, food and other amenities of being with a family, plus the learning of local language and culture.

Comes from the French au pair, meaning "on par" or "equal to". So the relationship with the person is on equal terms, he/ she is member of the family not a servant. Though the relationship is supposed to be temporary, its expected that they will remain in touch and on good terms years after the 'au pair' has left for his home country.

Example
One memory of that visit was of Jackie, the plump and pretty au pair at 18 Queens Road, who was huggable and on whom I had a crush.
Two Lives, by Vikram Seth

Shellacking

To beat decisively
To coat with 'lac'

Examples:
... Schwartz helped the Titans defeat Pittsburgh, Baltimore, Cleveland and Cincinnati. Included in that run was a 31-14 late-December shellacking of the Steelers.

Busking

Performing (usually music) for free at public places, aiming to get tips. Like you would see being done at train stations etc.

At least in London 'Tube' I know that proper license if given for busking and the 'Tube' administration even recommends giving tips to buskers. And mostly I have found these artists to be talented enough to deserve some tips and respectful listening.

One interesting case, that I got to know recently is from New York, where a celebraty Violanist played as an incognito busker and got 32 dollars after performing about an hour. Teaches something about perceptiont and shows how much beauty we ignore while rushing through our rut race. Read on Pearls Before Breakfast.

Recce

Recce: Short form of reconnaissance, specially the verb form of reconnaissance.

Example
The police also revealed that one of those arrested had allegedly done a recce of one of Gandhi’s lawyers house in the Capital.
HT news about underworld shooters' arrest in New Delhi

Caterwaul

To wail like like a cat.
To fight like screeching cats, having noisy arguments etc

Example
As always, he caterwauled at the sheer size of the shocking verdict, and blamed it on the undue passion created by Baker's attorneys that inflamed the jurors.
The Appeal, by John Grisham

To take a rain check

Background: Suppose you bought costly ticket to a match and then it got canceled due to rain. To save you from the loss of money, the match organizer can give you ticket-stub along with the ticket- the stub can be used as ticket when the 'replacement match' is played.

So now, taking a rain check would mean opting to leave main opportunity but still keep the option of availing it in future.

Another meaning of the term is option given by companies to avail of their promotion (sale, free-gift etc.), when they run out of stock.

Examples
- Dinner at your place this weekend? I hope I can take a rain check.
- During the 2007 Christmas season, demand for Nintendo Wii was so strong that Nintendo was forced to issue rain checks to customers
from book Strategic Innovation: New Game Strategies for Competitive Advantage

Sow your wild oats

Read the term in Neil Strauss' The Game- "The pickup thing was never about sowing my wild oats". As one can understand from the worrding itself, it has something to do with 'spreading seeds' :).

Roughly speaking it means-
to have wild sex life,
to have sexual relations with multiple partners,
to have one's fling of affairs,

I think the wording itself makes it clear that the term is intended more for males. The term is mostly used for indicating a young male's fun-filled life, before he gets married.

There is exact idiom for this 'phenomenon' in French- "jeter sa gourme". Trying to dissect this one literally, one understands the meaning much more graphically.