Coomer, n. is internet slang for a male who is addicted to masturbating, or, by extension, any excessively horny male.
The term originally derived from the "Coomer" meme first popularized on 4chan, featuring the Wojak character with a massively oversized right arm overdeveloped due to constant masturbating.
The term itself is a portmanteau of "coom" and "boomer," combining "coom," a jocular pronunciation of "cum," and the -oomer ending often attached to Wojak characters, originally derived from "boomer," short for "baby boomer."