Learn English – Why did jazz musicians start referring to an engagement as a “gig”

Kid as young goat is from the 1200s with

Extended meaning of "child" first recorded as slang 1590s, established in informal usage by 1840s.

Dictionary.com defines kid as (informal) child.

You would use it in direct conversation with persons you know well

"Does your kid collect stamps?"

although I don't see it as too informal to ask someone, "Any kids?" instead of, "Any children?"

Google-fu

The most well known is arguably Google-fu which is first in Usenet on 27th September 2002 in a comp.sys.mac.advocacy post by James Boswell:

Have you ANY idea how much a Wildcat 6210 costs? ( http://www.3dlabs.com/product/wildcatIII_6210_index.htm )

* google fu *

http://www.amazoninternational.com/html/hardware/pricing/graphics/pro...s_cards.asp
£1900 + VAT (that's 17.5% here)

Followed soon later on 29th October 2002 in rec.music.christian:

hope that helps. and practice your google fu, young one. ;)

And on 19th December 2002 in alt.arts.poetry.comments:

I had gavagai told to me in an intro philosophy or an intro linguistics class. wtf? is my google fu so much better than yours, grasshopper?

There's approximately 32 results in 2003, ~121 in 2004, ~279 in 2005, ~318 in 2006 and ~410 in 2007.

Code-fu

13th July 2000 in rec.arts.anime.misc:

Luckily, I'm one of those UNIX/Web developer/code fu types that one of the other posters referred to, so the money isn't really an issue for me.

2000: ~4 results, 2001: ~3 results, 2002: ~4 results, 2003: ~4 results, 2004: ~12 results, 2005: ~32 results, 2006: ~46 results, 2007: ~94 results.

UNIX-fu

29th May 1998 in aus.computers.sun:

Plus I like BSD-style unixes at home and sys v-style at work. Keeps me on my toes. good UNIX fu. {insert appropriate sound effects here}

Java-fu

18th November 1998 in alt.motd (from Soda.csua.berkeley.edu motd):

_ your Java fu is weak, grasshopper. Use a JIT.

"Your [X]-fu is weak, grasshopper" is a common pattern and may be the source.

Your kung fu is weak, grasshopper

This seems to be inspired by the 1972–1975 television series Kung Fu. The protagonist Kwai Chang Caine often had flashbacks to his childhood lessons, when his kung fu master called him Grasshopper. This is the source of other similar phrases such as "patience, grasshopper", and from the pilot):

Master Po: Close your eyes. What do you hear?
Young Caine: I hear the water, I hear the birds.
Po: Do you hear your own heartbeat?
Caine: No.
Po: Do you hear the grasshopper which is at your feet?
Caine: Old man, how is it that you hear these things?
Po: Young man, how is it that you do not?

Listen to a .wav file.