Adverb (fukushi), Adverb taking the 'to' particle, Suru verb
sobbing; weeping; whimpering
Onomatopoeic or mimetic word
いつまでメソメソしてるんだ。過ぎてしまったこと今更どうにもならないだろう。How long are you going to keep whimpering about that? You can't change the past.
Also written めそめそ