act up

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verb

acted up; acting up; acts up

intransitive verb

1

: to act in a way different from that which is normal or expected: such as

a

: to behave in an unruly, recalcitrant, or capricious manner

The children were acting up.

b

: show off

c

: to function improperly

This keyboard is acting up again.

2

: to become active or acute after being quiescent

Her rheumatism started to act up.

ACT UP

2 of 2

abbreviation

ˈakt-ˈəp 

AIDS Coalition to Unleash Power

Synonyms of act up

Verb

• act out
• misconduct
• disobey
• carry on
• run riot

Examples of act up in a Sentence

Verb

the two-year-old was acting up in church so much that her dad had to take her outside

you should have seen the boys acting up when they put on their aprons

Recent Examples on the Web

Verb

Likewise, when the young men started acting up, Josephine Holt took a keen interest in her newspaper. —Literary Hub, 21 Jan. 2026

If your trackpad is acting up, keyboard shortcuts can keep you productive without fighting the cursor. —Ken Colburn, AZCentral.com, 19 Jan. 2026

The resident followed this act up by going back inside and reemerging with a rifle, Evans said. —Timia Cobb breaking News Reporter, Dallas Morning News, 15 Jan. 2026

Some of the biggest names in Hollywood weren’t afraid to act up on Friday night. —Matt Donnelly, Variety, 10 Jan. 2026

Word History

First Known Use

Verb

1879, in the meaning defined at sense 1

Rhymes for act up

• backup
• blowup
• breakup
• buildup
• checkup
• cleanup
• holdup
• hookup
• lineup
• lockup
• makeup
• markup

Kids Definition

act up

verb

: to act in an unruly, abnormal, or annoying way