Tipping

The tip is just that. It's a little thing, etymologically the diminutive of 'tap' (in the sense of a something that stops up or delivers a stopping impulse). Over time, the sense of 'little thing' came to dominate, and perhaps the sense of 'little but important thing'.

Nowadays, people speak of 'tipping points'. That term has always amused me. I used to think of restaurants, hotels, and other service-related institutions as 'tipping points'. You go there, you give tips. You top up with a tip up. Haha.