Poor Dewi Lingo! Not only is he the shortest boy in the class, but . . . Is Welsh gwlyb, then, used with the transferred meaning 'feeble and unimpressive' just as 'wet' is in [slang] English?
No, not really I've never heard anyone translating that idiom into Welsh. This sentence simply refers to physical state, ir poor Dewi Lingo being the wettest of a group who are all wet from rain or whatever.
Thank you! I see my idea was over-ingenious. Sometimes, to paraphrase Freud, an adjective is just an adjective.