This is an abbrevation for "you amuse me" for those jokes that make you smile but not actually laugh out loud.
Friend= "How many racists does it take to screw in a lightbulb? .... None -- they don't want to be enlightened!
*friend laughs uproariously*
You sigh then say... "YAM"
*friend laughs uproariously*
You sigh then say... "YAM"
by The Tall Leggy Blonde July 08, 2009