mesugaki
Literally, "bitch brat". A young girl who acts sassy and provocative towards adults in a sexually charged manner. Often qualifies for loli. The masculine equivalent is called "osugaki".
Typical attributes include smug or naughty expressions, a mocking laugh, a single fang / skin fang, twintails, short shorts, miniskirts/microskirts, exposed midriff, calling people "zako", etc. May be an oppai loli.
Not to be confused with erogaki ("erotic brat") who is sexually forward or seductive without being sassy, or kusogaki ("shitty brat") who is sassy without a sexual undertone.
The present meaning seems to date back to August 2018, after the announcement of Grim Aloe being added to Bombergirl. For more details, see Danbooru comment #2388484.