1
general::
Barely, scarcely, just within the limit, as in This book will be finished just under the wire. This term comes from horse racing, where the wire marks the finish line. [First half of 1900s] Also see DOWN TO THE WIRE.
American Heritage Idioms
2
general::
Phrase(s): under the wire
Fig. just barely in time or on time. • I turned in my report just under the wire. • Bill was the last person to get in the door. He got in under the wire.
McGrawhill's American Idioms And Phrasal Verbs