Definition of "buckler"

Bucklers will be assigned to buckle up drivers in the morning and make sure they stay buckled up.

1986, Press Summary - Illinois Information Service, page 6724

I am eight times thrust through the doublet, four through the hose, my buckler cut through and through; my sword hacked like a hand-saw -- ecce signum!

c. 1597 (date written), William Shakespeare, “The First Part of Henry the Fourth, […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act II, scene iv], line 166

The target or buckler was carried by the heavy armed foot, it answered to the scutum of the Romans; its form was sometimes that of a rectangular parallelogram, but more commonly had it's bottom rounded off; it was generally convex, being curved in it's breadth.

1786, Francis Grose, A Treatise on Ancient Armour and Weapons, page 22

Most zeids have spiny projections or bucklers at the dorsal- and anal-fin bases and bony plates on the ventral surface of the abdomen.

2000, Jeffrey Martin Leis, Brooke M. Carson-Ewart, The Larvae of Indo-Pacific Coastal Fishes, page 186

Can Oxford, that did ever fence the right, / Now buckler falsehood with a pedigree?

c. 1591–1592 (date written), William Shakespeare, “The Third Part of Henry the Sixt, […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act III, scene iii]