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1.
an invitation for sex
in 'polite' grammatical form "lets fuck" would be
"say,we should have sexual intercourse"
man: heres the pizza you ordered
woman: i didnt order a pizza
man: lets fuck!
by Haberdasher December 11, 2005
 
2.
when you get with someone of the opposite sex and the guys penis goes into the womans vagina
Hey sexy, lets get together at my house tonight and lets fuck
by bob March 09, 2005