Close and lock
Fasten up means to secure something by closing or tightening its fasteners—buttons, zips, straps, or buckles: Fasten up your coat, it's freezing out there.
The phrase emphasizes completeness, suggesting all fasteners rather than just one.
It's essentially interchangeable with fasten but slightly more emphatic about finishing the job.
IDEA.org / Words / fasten up
Close and lock
Fasten up means to secure something by closing or tightening its fasteners—buttons, zips, straps, or buckles: Fasten up your coat, it's freezing out there.
The phrase emphasizes completeness, suggesting all fasteners rather than just one.
It's essentially interchangeable with fasten but slightly more emphatic about finishing the job.
Related — lace up (Fasten with cords); buckle up (Secure your restraint); strap in (Fasten a safety restraint); attach · tighten up · do up · button up · button · clasp · zip · nail · secure · tie
SECURING/ATTACHMENTfasten up ≠ unfasten Release from attachment; undo · open · unbuckle · unbutton · unzip
TIGHTNESS/TENSIONfasten up ≠ loosen Reduce tension or grip; slacken · loosen up · let out · relax · ease
fasten, fast, fastened, fastening, fastener, fasteners, fastenings, fastens, unfasten
Word data: Linguabase (CC0) · www.idea.org/words/fasten_up