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People in the Undertale fandom. They tend to be made fun of for being butthurt and cringe-worthy. Among other things, they are also annoying.
Hunter: Undertale sucks.

Underfag: Hey!! Undertale is good the fandom just sucks but Undertale is good and has good ships lol XDD!! Everyone has their own opinion!
Hunter: Then let me share mine.

Underfag: You're just being stupid let me have my own opinion!

Preston: They took the bait.
by IgnitingInsanity May 26, 2016
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Someone who is waay to obsessed with Undertale. They often say "I'm filled with determination" or make skeleton jokes. Like the character, sans does. They also bring up Undertale way too much.
Friend 1: Wow dude, that episode truly was sans-sational!
Friend 2: Stop being such an underfag.
by DerpRunner October 07, 2016
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Any Undertale Player/Fanboy that meets the following Criteria.

1) Is obsessed and will go to any length to get the merchandise .
2) Is under the age of 10.
Guy A: God Damnit
Guy B: What?
Guy A: My little brother is such an Underfag
by ExtraShard July 04, 2016
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People who are obsessed with the indie game Undertale by Toby Fox. They have a horrible tendency to say things like "welp", "hoi", "boi", etc. They are also noted to be extremely toxic and get butthurt incredibly fast, this us due to most of the fandom being nercophilliac 12 year olds who lashes out on you saying things like "l0l uR gOnNa haVe a Bad TiMe!!1!11!!".
Underfags are so annoying.
by Buffsuki7 June 16, 2018
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