keep tabs on someone Definition

tab¹
noun

1. A small flap, tag, strip of material, etc attached to an article, for hanging it up, opening, holding or identifying it, etc.

Thesaurus: tag, sticker, flag, label, marker.
2. A small strip of material attached to a garment for hanging it up.
(chiefly N Amer)
3. A bill, eg in a restaurant, especially an account that is opened, eg at the beginning of an evening, and added to until it is settled at the end of the evening.
(chiefly N Amer)
4. A price; cost.
(N Eng)
5. slang
A cigarette.

verb

tabbed, tabbing
1. To fix a tab to something or someone.

Idiom: keep tabs on someone or something

colloq
To keep a close watch or check on them or it.

Idiom: pick up the tab

(hiefly N Amer)
To pay the bill.

Etymology: 19c: probably originally a dialect word.