Onacups are masturbation toys for men. The term originated in Japan, and most such toys are manufactured there.
The name "onacup" is derived from the word onanism (which means masturbation) combined with "cup", as the design of the toys resembles a plastic cup. It also sounds similar to "onna", a word meaning "woman".
Old-style onacups are typically made in a plastic "cup" case with a picture of an attractive anime girl on the front. Inside, there is a soft jelly sleeve designed with nubs and other features to increase stimulation. Lubricant is also already inside.
New-style onacups have a more stylish design with no anime girl, and are designed to recreate various sexual acts such as fellatio and anal sex in addition to vaginal intercourse.
Onacups are usually purchased at adult stores in Japan. A user takes off the sealed lid and inserts his erect penis into the hole. After masturbating and ejaculating into the cup, it can be thrown away or washed out.