fat-mouth1
US slang
noun
One who talks extravagantly, a loudmouth.
Origin
1940s; earliest use found in Lester V. Berrey (b. 1907), lexicographer.
fat-mouth2
US slang (chiefly Black English )
verb
1 [no object] To talk a great deal about something, with little or no action or result.
2 [with object] To talk excessively to, especially in order to cajole or sweet-talk.
Origin
1970s; earliest use found in The Atlantic Monthly.