The term has a pretty nasty etymology. It refers to sexual intercourse with one partner directly after sexual intercourse with another partner — the indication is that the receiving partner is still "sloppy" from the previous interaction. Ick factor aside, it's one of many derogatory slang words that make the woman an object. "Sloppy seconds" is a way of saying, "I already marked that territory," which by extension makes the woman a territory or possession. Brian's use of the term is more innocent, but the implications remain. He kissed Lindsay first, therefore she is his property.