Etymology
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glom (v.)
1907, glahm "grab, snatch, steal," American English underworld slang, from Scottish glaum (1715), apparently from Gaelic glam "to handle awkwardly, grab voraciously, devour." Sense of "look at, watch" (1945) apparently is derived from the same source. Related: Glommed; glomming.
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glomeration (n.)

"accumulation; ball," 1620s, from Latin glomerationem (nominative glomeratio), noun of action from past-participle stem of glomerare "to wind or form into a ball, roll together, collect," from glomus "ball of yarn, ball-shaped mass," from Proto-Italic *glemos-, from PIE *glem- or *glom-, perhaps originally "ball," but the reconstruction is uncertain (see glebe).

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gloaming (n.)
Old English glomung "twilight, the fall of evening," found but once (glossing Latin crepusculum), and formed (probably on model of æfning "evening") from glom "twilight," which is related to glowan "to glow" (hence "glow of sunrise or sunset"), from Proto-Germanic *glo- (see glow (v.)). Fell from currency except in Yorkshire dialect, but preserved in Scotland and reintroduced by Burns and other Scottish writers after 1785.
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gloom (n.)
1590s, originally Scottish, "a sullen look," probably from gloom (v.) "look sullen or displeased" (late 14c., gloumen), of unknown origin; perhaps from an unrecorded Old English verb or from a Scandinavian source (compare Norwegian dialectal glome "to stare somberly"), or from Middle Low German glum "turbid," Dutch gluren "to leer." Not considered to be related to Old English glom "twilight" (see gloaming).

Sense of "darkness, obscurity" is first recorded 1629 in Milton's poetry; that of "melancholy, dejection, cloudiness or cheerless heaviness of mind" is from 1744; but gloomy with a corresponding sense is attested from 1580s.
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glebe (n.)

late 14c., "soil of the earth; cultivated land;" also "a piece of land forming part of a clergyman's benefice," from Old French glebe, from Latin gleba, glaeba "clod, lump of earth," possibly from a PIE *glem- or *glom-, which might mean "contain, embrace" or "ball," or might be two different roots. Possible cognates include Old English clamm "a tie, fetter;" Old High German klamma "trap, gorge;" Old Irish glomar "gag, curb;" Latin globus "sphere," gleba, glaeba "clod, lump of earth;" Old English clyppan "to embrace;" Lithuanian glėbys "armful," globti "to embrace, support."

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clam (n.)

bivalve mollusk, c. 1500 (in clam-shell), originally Scottish, apparently a particular use of Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (source also of Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"), possibly from a PIE *glem- or *glom- "contain, embrace" (see glebe).

If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.

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