) military slang
: Lieutenant Colonel
Calling a lieutenant colonel a "light" colonel--implying a lack of weight to throw around--may stem from the abbreviation for the rank in the English language: LtC or (archaic) LtCol. Phonetically, it's not much of a leap. The nickname also helps distinguish a light colonel from a "full bull," "full bird," O6--a "full" colonel.
However, one doesn't address an O5 as "Light Colonel Remington," though; it's improper to the point of being rude, particularly for subordinates. I can't stress this enough--it's simply not done. You call a light colonel "Colonel" or "Lieutenant Colonel" or "Sir" or "Ma'am". Just like "veep," it's a slang term for clarification when the subject isn't around:
"Did you see Colonel McGinty got command of the new wing?"
"He's just a light colonel, right?"
"Yeah, but he's a fast burner--that's how he got a wing! He'll make full bird this year."