When a male or female is just attractive enough that you would potentially shake you're hand in the "so-so" action. This being when your hand is held out and rocked back and forth to give an "okay or mediocre" judgment rather then a strong approval or dis-approval.
When someones looks are neither good nor bad.
"What did you think of that girl you met last night at the party?" "Eh...I'd shake a hand at it."