From Wiktionary, the free dictionary
From в (v) + во́лю (vólju), accusative of во́ля (vólja, “will”); approximate sense "to (your) will".
вво́лю • (vvólju) (diminutive вво́люшку)
- (colloquial) (in) plenty, as much as is wanted, a lot
- Synonyms: вдо́воль (vdóvolʹ), всласть (vslastʹ)
- есть вво́лю ― jestʹ vvólju ― to eat plenty, to eat one's fill