Middle English smok, "women's undergarment, shift," from from Old English smoc "garment worn by women," corresponding to the men's shirt, from Proto-Germanic *smukkaz (source also of Old Norse smokkr "a smock," but this perhaps is from Old English; Old High German smoccho "smock," a rare word; North Frisian smok "woman's shift," but this, too, perhaps is from English).
Klein's sources, Barnhart and the OED see this as connected to a group of Germanic sm- words having to do with creeping or pressing close, such as Old Norse smjuga "to creep (through an opening), to put on (a garment)," smuga "narrow cleft to creep through; small hole;" Old Swedish smog "a round hole for the head;" Old English smugan, smeogan "to creep," smygel "a burrow." Compare also German schmiegen "to cling to, press close, nestle;" and Schmuck "jewelry, adornments," from schmucken "to adorn," literally "to dress up."
Watkins, however, traces it to a possible Germanic base *(s)muk- "wetness," figuratively "slipperiness," from PIE root*meug- "slimy, slippery" (see mucus). Either way, the original notion seems to have been "garment one creeps or slips into," by the same pattern that produced sleeve and slip (n.2).
In the original sense it has largely been replaced by euphemistic shift (n.2). Until 18c. smock was so much the common word for a woman's undergarment that it could be emblematic of womanhood generally, as in verb smock "to render (a man) effeminate or womanish" (1610s); smocker "man who consorts with women" (18c.). Smock also was an adjective, "belonging or relating to women;" smock-face "person having a pale, effeminate face" is attested by c. 1600 (Dryden's Endymion was a smock-fac'd boy). Smell-smock "licentious man" was in use c. 1550-c. 1900.
The modern meaning "woman's or child's loose dress or blouse" is from 1907; the sense of "loose garment worn by artists over other clothes" is from 1938. A smock-race (1707) was a foot-race for women and girls, an old country pastime:
Smock Races are commonly performed by the young country wenches, and so called because the prize is a holland smock, or shift, usually decorated with ribbands. [Joseph Strutt, "Sports and Pastimes of the People of England," 1867]