the.com/grower

means A person or thing (often said of a man's anatomy) that starts small but increases notably in size once aroused or active — as opposed to a 'shower,' which looks the same throughout.

from Straightforwardly built from 'grow' plus the agent suffix '-er' — someone or something that grows. The word itself is old and plain; its slang sense as half of the 'grower vs. shower' pairing is a much more recent bit of locker-room taxonomy, first circulating in spoken slang before showing up in print and online discussion, with no fancier origin than people needing a quick way to describe size at rest versus size engaged.