1856, from un- (2) "reverse, opposite of" + balance (v.).

"either of the dishes of a balance," 1830, from scale (n.2) + pan (n.).

1895, from im- "not" + balance (n.).

also hammer-head, 1560s, "head of a hammer," from hammer (n.) + head (n.). From 1796 (American English) in reference to a kind of shark, so called for its broad, transverse head. The animal is referred to as hammer-headed shark from 1752 and hammer-fish from 1745. The older name for it was balance-fish; there was a full specimen and a head of another under that name in the Royal Society Museum by 1681:

He hath his Name not unaptly from the ſhape of his Head, very different from that of all other Fiſhes, being ſpread out horizontally, like the Beam of a Balance; his eyes ſtanding at the two extremes, as the iron Hooks do at the end of the Beam. He grows sometimes to the length of four or five yards: but this is a young one. [Nehemiah Grew, M.D., "Catalogue & Deſcription Of the Natural and Artificial Rarities Belonging to the Royal Society And preſerved at Greſham Colledge. Whereunto is Subjoyned the Comparative Anatomy of Stomachs and Guts. By the ſame author" London, 1681 ]

Compare French requin marteau, Italian pesce martello, etc. The Greeks named it for the cross-bar of a yoke (zygon) and called it zygania. "According to Xenocrates, and to Philotinus ap. Galen vi. 727, it is tough and indigestible, but may be eaten pickled" [Thompson].

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"horizontal bearing beam," late 13c., from Anglo-French sumer, Old French somier "main beam," originally "pack horse," from Vulgar Latin *saumarius, from Late Latin sagmarius "pack horse," from sagma "packsaddle" (see sumpter).

1620s, from well (adv.) + past participle of balance (v.).

"having rays, furnished with rays or ray-like parts, shining," 1660s, from Latin radiatus, past participle of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius).

"ray of light from the moon," 1580s, from moon (n.) + beam (n.).

1590s, "in equilibrium," past-participle adjective from balance (v.). In reference to meal, diet, etc., by 1908.

"long pole," 1640s, specifically, "long spar run out from a ship" (1660s), from Scottish boun, borrowed from Dutch boom "tree, pole, beam," from a Middle Dutch word analogous to German Baum, English beam (n.). As "movable bar for a microphone or camera," 1931.