While it would be nice to be able to go back to English common law to dig up this one, the Internet seems to think the reason “contract” is abbreviated in law with a “K” is because the word contract begins with a hard “C” which is more akin to a “K” visually.

• C = “see”
• K = “kay”

Other seemingly nonsensical uses for the letter “K” in abbreviation:

• K = From baseball, meaning strikeout (because S was already taken for sacrifice)
• EKG = Electrocardiogram (from the German elektrokardiogramm)
• K = potassium (from the Latin kalium)
• k = spring constant

From the Internet hivemind.