Etymology
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somato- 

before vowels somat-, word-forming element meaning "the body of an organism," from combining form of Greek sōma (genitive sōmatos) "the body, a human body dead or living, body as opposed to spirit; material substance; mass; a person, human being; the whole body or mass of anything," a word of uncertain origin. Perhaps originally "compactness, swelling," and from PIE root *teue- "to swell."

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astro- 

element active in English word formation from mid-18c. and meaning "star or celestial body; outer space," from Greek astro-, stem and combining form of astron "star," which is related to aster "star," from PIE root *ster- (2) "star." In ancient Greek, aster typically was "a star" and astron mostly in plural, "the stars." In singular it mostly meant "Sirius" (the brightest star).

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-ly (1)

suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).

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-ly (2)

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.

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