Etymology: cucumber +‎ -ish
Adjective: cucumberish (comparative more cucumberish, superlative most cucumberish).
(informal) Resembling or characteristic of cucumber."

Cucumberish is a bit of Victorian slang that was first recorded 1822.1 It was used exclusively to refer to people who were cool, in the sense of cool as a cucumber -- that is to say, icy, chill in manner, unimpressed, unexcitable, unfriendly, and generally underwhelmed.

If you ever thought that this sort of twee linguistic roundaboutation was invented for the sake of Bertie Wooster, rest assured, it was not. Real people really said these things.

These days, of course, Wikipedia pretty much covers the common usage, bless them. People really do use the word to talk about plants, tastes, and smells that are cucumber-like. However, the superlative "most cucumberish" has never been used by anyone except when making the "cool as a cucumber" allusion.

1. Life in London by Pierce Egan, Robert Cruikshank, and George Cruikshank: this is an annoying work that appears to exist only so that the authors can wedge every slang word they know into something like a narrative. It is somewhat useful from an etymological standpoint, but SO OBNOXIOUS.