slither v.

Middle English sliden, "glide, move smoothly and easily over a surface," also "to fall, lose one's balance through slipping," from Old English slidan (intransitive, past tense slad, past participle sliden) "to glide, slip, fall, fall down;" figuratively "fail, lapse morally, err; be transitory or unstable," from Proto-Germanic *slidanan "to slip, slide" (source also of Old High German slito, German Schlitten "sleigh, sled"), from PIE root *sleidh- forming words for "to slide, slip; slippery" (source also of Lithuanian slysti "to glide, slide," Old Church Slavonic sledu "track," Greek olisthos "slipperiness," olisthanein "to slip," Middle Irish sloet "slide").

The meaning "lose one's balance through slipping, lose one's footing" is attested from early 13c. (for distinction from slip, see below). The transitive sense of "cause to glide or move along a surface" is from 1530s. The meaning "pass gradually from one state or condition to another" is from late 14c. Related: Slid; slidden; sliding.

The phrase let (something) slide "let it take its own course, take no consideration of" is in Chaucer (late 14c.) and Shakespeare. Sliding scale in reference to payments, etc., varying under certain conditions is from 1842.

We slide or slip on a smooth surface : we slide by intention ; we slip in spite of ourselves. In the Bible slide is used for slip. Slide generally refers to a longer movement : as, to slide down hill ; to slip on the ice. We glide by a smooth and easy motion, as in a boat over or through the water. [Century Dictionary]

Old English gadrian, gædrian "unite, agree, assemble; gather, collect, store up" (transitive and intransitive), used of flowers, thoughts, persons; from Proto-Germanic *gaduron "come or bring together, unite" (source also of Old English gæd "fellowship, companionship," gædeling "companion;" Middle Low German gadderen; Old Frisian gaderia; Dutch gaderen "to gather," gade "spouse;" German Gatte "husband;" Gothic gadiliggs), perhaps from PIE *ghedh- "to unite, join" (see good (adj.)). Change of spelling from -d- to -th- is 1500s, reflecting earlier change in pronunciation (as in  father). Related: Gathered; gathering.

"to slide, slip," from Old English asliderian, from slider "slippery," from the source of slide (v.); also compare slither. Related: Sliddery.