father-in-law
舅 centers on a male relative by marriage: specifically one's father-in-law.
舅 combines 臼 (mortar) and 男 (man). The historical development is uncertain, but it likely relates to a male relative through a phonetic or semantic association.
The top 臼 looks like a mortar, and the bottom 男 is a man. Imagine a father-in-law grinding rice in a mortar to prepare a family meal.
For キュウ, picture a father-in-law giving you a cute little mortar as a gift: cute -> キュウ, and the mortar 臼 reminds you of the kanji.