slackyoudaoicibaDictYouDict[slack 词源字典]
slack: [OE] In common with Dutch and Swedish slak, slack comes from a prehistoric Germanic *slakaz. This was derived from the same ultimate source that produced Latin laxus ‘loose’ (source of English lax, relax, release, and relish) and languēre ‘languish’ (source of English languish). The plural noun slacks was first used for ‘trousers’ in the early 19th century. (The noun slack ‘small pieces of coal’ [15] is a different word, probably borrowed from Middle Dutch slacke ‘waste produced by smelting metal’.)
=> languish, lax, relax, release, relinquish[slack etymology, slack origin, 英语词源]
pedal (n.)youdaoicibaDictYouDict
1610s, "lever (on an organ) worked by foot," from French pédale "feet, trick with the feet," from Italian pedale "treadle, pedal," from Late Latin pedale "(thing) of the foot," neuter of Latin pedalis "of the foot," from pes (genitive pedis) "foot," from PIE root *ped- (1) "a foot" (see foot (n.)).

Extended to various mechanical contrivances by 1789. Pedal steel guitar is from 1969. Pedal-pushers "type of women's trousers suitable for bicycling" is from 1944.
When college girls took to riding bicycles in slacks, they first rolled up one trouser leg, then rolled up both. This whimsy has now produced a trim variety of long shorts, called "pedal pushers." ["Life," Aug. 28, 1944]
slack (n.1)youdaoicibaDictYouDict
early 14c., "cessation" (of pain, grief, etc.), from slack (adj.). Meaning "a cessation of flow in a current or tide" is from 1756; that of "still stretch of a river" is from 1825. Meaning "loose part or end" (of a rope, sail, etc.) is from 1794; hence figurative senses in take up the slack (1930 figuratively) and slang cut (someone) some slack (1968). Meaning "quiet period, lull" is from 1851. Slacks "loose trousers" first recorded 1824, originally military.
slacker (n.)youdaoicibaDictYouDict
popularized 1994, but the meaning "person who shirks work" dates to 1897; agent noun from slack (v.). In early use also slackster (1901). Compare Old English sleacornes "laziness," which is not, however, an agent noun. Related: Slackerly; slackerish.