slide-rule (n.)

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]

c. 1200, "principle or maxim governing conduct, formula to which conduct must be conformed" from Old French riule, Norman reule "rule, custom, (religious) order" (in Modern French partially re-Latinized as règle), from Vulgar Latin *regula, from Latin regula "straight stick, bar, ruler;" figuratively "a pattern, a model," related to regere "to rule, straighten, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").

By mid-14c. as "control, government, sway, dominion." The meaning "regulation governing play of a game, etc." is from 1690s; the phrase rules of the game is by 1787. To bend the rules "interpret leniently, overlook infringement" is by 1680s.

The meaning "strip with a straight edge used for making straight lines or measuring" is from mid-14c. Typography sense of "thin strip cut type-high and used for printing continuous lines" is attested from 1680s. Rule of law "supremacy of impartial and well-defined laws to any individual's power" is from 1883. Rule of the road in reference to the fixed customs, formerly much varying from country to country, which regulate the sides to be taken by vehicles in passing each other, is by 1805.

The rule of the road is a paradox quite,

In driving your carriage along,

If you keep to the left you are sure to go right,

If you keep to the right you go wrong.

[Horne Tooke, "Diversions of Purley," 1805]