on‧to S1 W2 (also on to) /ˈɒntə; before vowels ˈɒntʊ; strong ˈɒntuː $ ˈɑːn-, ˈɒːn-/ preposition

1. used to say that someone or something moves to a position on a surface, area, or object:
She watched him walk onto the platform.
Don’t jump onto (=into) the bus while it’s moving.
Pour the syrup on to the egg mixture.
The car rolled over onto its side.
down/out/up etc onto something
Let’s get back onto the highway.

2. used to say that a room, door, or window faces towards something or allows movement into another place:
The dining room looks out onto a pretty garden.
a gate leading on to a broad track

3. be onto somebody informal
a) (also get onto somebody especially British English) to speak to someone in order to tell them or ask them something:
A number of people have been onto me complaining about the noise.
Get onto the Press Office and find out what’s happening.
b) to know that a particular person did something wrong or committed a crime:
The police are onto him.

4. be onto something informal
a) to have discovered or produced something new and interesting:
With the new show, we were onto something big.
be onto a good thing/a winner
I think she’s onto a real winner with this song.
b) (also get onto something) to be dealing with something or start dealing with something:
I’ll get onto it right away.

[TahlilGaran] Dictionary of Contemporary English ▲

to be onto someone
to have discovered the truth about someone or something

[TahlilGaran] English Idioms Dictionary ▲