A person who is very smart (or book smart) but you wouldn't know it by looking at them because they're pretty much the opposite of every stereotype there is about intelligent people.
There are three tiers of smartshmallowdom:

Tier 1: THE COOL SMARTSHMALLOW. This is the good kind of smartshmallow, the kind you actually want to be around. They have a fun sense of humor and/or a really good sense of style as well as mad skillz. Possibly a party animal as well.

Tier 2: THE DITZY SMARTSHMALLOW. This is the kind of smartshmallow who, despite their high IQ/GPA, insists on saying "like" or "dude" after every 5 words they say and often overuses all the really annoying slang terms. Basically, they are the embodiment of the "dumb blonde" stereotype (not saying all blondes are dumb, just referring to the stereotype here).

Tier 3: THE ASSHOLE SMARTSHMALLOW. This is the worst kind of smartshmallow. They are not only idiotic, annoying, and do stupid things, but they are also really mean to you or other people and you can't even say to yourself "It's okay, I'll be successful 10 years from now and they'll be working at McDonald's" because THEY ARE SMARTER THAN YOU.
by Hassan & Fei Yen November 19, 2013
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