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Malfunction, as in I’m not sure what’s wrong with my car, but the transmission is acting up. In both usages up means “abnormally.”
In addition to the idioms beginning with act
- act of faith
- act of God
- act on
- act one’s age
- act out
- act up
- act upon
also see:
- catch in the act
- clean up (one’s act)
- do a disappearing act
- get in the act
- get one’s act together
- hard (tough) act to follow
- high-wire act
- in the act of
- put on an act