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Glite Dictionary
up to something

up to something

19 7.0
Doing something secret that may be wrong or harmful
  • idiom
  • Informal
translation icon : estar tramando algo
  • I knew he was up to something when he started acting so secretive.
  • think he's up to something
  • know he's up to something
  • up to something big

Whatever he is up to, it does sound pretty fun.

Examples

Synonyms

to lurk
vsup to something
104 5.5

To stay hidden and move secretly to watch or frighten people

describes secret action as an activity rather than simply hiding or watching
up to no good
vsup to something
  • Informal
19 7.4

Acting in a secret or suspicious way to do something wrong

is secretive and may be only mischievous rather than clearly dishonest
up to some devilment
vsup to something
  • UK
9.6

Doing small naughty things for fun

is more secretive and can seem suspicious rather than openly playful
up to mischief
vsup to something
  • Informal
3.9

Doing playful, slightly naughty things that are not serious

has more secret or possibly suspicious intent than mere play

Antonyms

innocent
∞ 2.2

Not guilty of a crime or wrongdoing

transparent
68 4.1

A person or process that is open and honest and does not hide information

How Suspicious

up to no good
  • Informal
19 7.4
up to something
  • Informal
19 7.0
suspicious
300 2.7
innocent
134 1.0

Surface Forms

up to something main

Morphology

up + to + something

The phrase relies on the idiomatic use of the particle sequence 'up to' meaning 'engaged in' combined with the vague pronoun 'something'; while there is a loose compositional cue (being 'up to' indicates involvement), the specific sense of secretive or suspicious activity is a figurative/idiomatic extension that is not fully predictable from the bare meanings of 'up', 'to', and 'something'. A B1 learner who knows the words may guess involvement but would likely need exposure to reliably infer the suspicious/mischievous nuance.

Etymology

Up to something may come from the image of a person climbing up into a hidden place to do something secret or quiet. So, when we say someone is up to something, we mean 'doing something secret, suspicious, or sneaky'.