Definition
To quit, to fold.
How People Actually Use It
To contribute money for a specific purpose or for the good of the group. Informal The phrase may derive from the act of passing around a hat, into which contributors would "throw in" some money.
""Another six-pack of Hamm's? I'll throw in for that!" Not: "There's orphans starving in Haiti? I'll throw in for that!" Situation too formal or grave."
Source: Urban Dictionary