Starting an inappropriate slow clap when she introduces you to her hot mother. TFM.

This is funny because it references the “slow clap”, a common cliche of 1980’s sports and teen comedy films whereby a character will begin clapping their hands in an unusually slow manner to indicate their strong approval of something, gradually speeding up in the expectation that others in the vicinity to eventually join in until the collective act crescendos into rapid and enthusiastic applause. This action is not considered an appropriate alternative to a handshake or other standard greeting one would give to a new acquaintance. By beginning a slow clap upon meeting his female companion’s attractive mother, the poster is displaying his dominant personality through the rejection of the common rules of decorum that govern the situation he finds himself in.