Litteraly: Subtle, difficult to detect, sneaky.
Often used with sarcasm - "easy to detect", "evident", touchless"
From the Venetian verb "sgammar", litteraly "to find someone out" and "un" negative.
If someone is doing something wrong hoping to be unseen but you and maybe others notice it, you say loud "unsgammable"
If you approach a girl with a bottle of Jägermeister is not unsgummable that you want her drunk.
If you come home 5 a.m. slamming the door and puking everywhere your drunkness is definetely sgummable!