Bottomless means so deep that no floor can be found. A bottomless pit, bottomless coffee, a bottomless appetite.
Nothing is literally bottomless, so the word is always an exaggeration, and a bottomless pit is a figure for anything that swallows money without filling. In American restaurants bottomless coffee means free refills, which is a cheerful modern use.
1adjectivebottomlessSo deep that no floor can be found.
“The old well seemed bottomless; we dropped a stone and never heard it land.”
bottomless is a quality.
Bottom plus -less, in Middle English, on the same pattern as endless and countless.
Middle English
botemles
English, today
bottomless
From Middle English botemles, botmeles, equivalent to bottom + less. Cognate with West Frisian boaiemleas (“bottomless”), Dutch bodemloos (“bottomless”), German Low German boddenloos (“bottomless”), German bodenlos (“bottomless”), Swedish bottenlös (“bottomless”), Icelandic botnlaus (“bottomless”).
Etymology from Wiktionary, CC BY-SA