From the Super Mario Wiki
Able Juice is an item used to cure an ally of any status ailments in Super Mario RPG: Legend of the Seven Stars. Able Juice comes in a blue cup with an "R" on it.
Status ailments, one way or another, weaken party members, making them less effective in battle. Able Juice can be used to cure these status ailments for one party member, making them "able" to fight again. This item can only be used during battle since all status ailments are eliminated at the end of any battle.
It can be bought for four Coins and sold for two Coins in the vast majority of shops throughout the game, starting from the first shop in the game at the Mushroom Kingdom. It can also be won from the Arachne, Carroboscis, Geckit, Guerilla, Heavy Troopa, Mr. Kipper, Rat Funk, and Stinger.