Shortening of capslock, itself a shortening of "capital letters lock". As the usage of capital letters online often carries the connotation of yelling or screaming (which can be
rude), leaving capslock on accidentally can lead to confrontation, hence "caps" to inform
people that you did it by accident. I have no clue why this shit has not been added yet considering how important it is to internet etiquette.
The phrase "caps" itself can be considered a shortening of "sorry, left capslock on". "sorry, caps" is also
common.