This term refers to fuckable elderly persons, esp. women.
The word is formed from a contraction of 'frail' and 'bait':
FRAIL adj. Having delicate health; not robust; weak.
BAIT n. An enticement; temptation.
See also jailbait
Man1 and Man2 are walking down the street. Hot granny crosses the road.
Man1: "Phewee, check out that frailbait!"
Man2: "Yeah! Imagine that bent over its Zimmer!"