just on Definition

on
prep

1. Touching, supported by, attached to, covering, or enclosing.

Example: a chair on the floor
Example: a dog on a lead
Thesaurus: upon, atop, against, near, above, next to, about, at; in contact with, touching, held by, supported by.
2. In or into (a vehicle, etc).

Example: got on the bus
3. colloq
Carried with (a person).
Example: I've got no money on me
4. Very near to or along the side of something.

Example: a house on the shore
5. At or during (a certain day or time, etc).

Example: on Monday
Example: on the hour
6. Immediately after, at or before.

Example: He found the letter on his return
7. Within the (given) limits of something.

Example: a picture on page nine
8. About.

Example: a book on Jane Austen
9. Towards.

Example: marched on the town
10. Through contact with or as a result of something.

Example: cut oneself on the broken bottle
11. In the state or process of something.

Example: on fire
Example: on a journey
12. Using as a means of transport.

Example: goes to work on the bus
13. Using as a means or medium.

Example: talk on the telephone
Example: a tune on the piano
14. On the occasion of something.

Example: shoot on sight
15. Having as a basis or source.

Example: on good authority
Example: arrested on suspicion
16. Working for or being a member of something.

Example: on the committee
Example: work on the case
17. At the expense of or to the disadvantage of something or someone.

Example: treatment on the National Health
Example: drinks on me
Example: the joke's on him
18. Supported by something.

Example: live on bread and cheese
19. Regularly taking or using something.

Example: on tranquillizers
20. In a specified manner.

Example: on the cheap
21. Staked as a bet.

Example: put money on a horse
22. Following.

Example: disappointment on disappointment

adverb

1. Said especially of clothes: in or into contact or a state of enclosing, covering, or being worn, etc.

Example: have no clothes on
2. Ahead, forwards or towards in space or time.

Example: go on home
Example: later on
3. Continuously; without interruption.

Example: keep on about something
4. In or into operation or activity.

Example: put the radio on

adj

1. Working, broadcasting or performing.

Example: You're on in two minutes
2. Taking place

Example: Which films are on this week?
3. colloq
Possible, practicable or acceptable.
Example: That just isn't on
4. colloq
Talking continuously, especially to complain or nag.
Example: always on at him to try harder
5. In favour of a win.

Example: odds of 3 to 4 on
6. cricket.
On the side of the field towards which the bat is facing, usually the batsman's left and the bowler's right. Opposite of off 6.

Idiom: be on to someone or something

To realize their importance or intentions.

To be in touch with them.
Example: We'll be on to you about the party on Saturday

Idiom: get on to someone

colloq
To get in touch with them.

Idiom: just on

Almost exactly.
Example: have collected just on £50

Idiom: on and off

Now and then; occasionally.
Thesaurus: sometimes, infrequently, now and then, from time to time, periodically.

Idiom: on and on

Continually; at length.

Idiom: on time

Promptly; at the right time.

Idiom: on to

To a position on or in.
Thesaurus: to, toward, adjacent to, against, upon, above, over.

Etymology: Anglo-Saxon.