Why is the ninth month called September?
early 14c., "to escape, to move softly and quickly," from an unrecorded Old English word or cognate Middle Low German slippen "to glide, slide," from Proto-Germanic *slipan (cf. Old High German slifan, Middle Dutch slippen, German schleifen "to glide, slide"), from PIE *sleib-, from root *(s)lei- "slimy, sticky, slippery" (see slime (n.)).
From mid-14c. with senses "lose one's footing," "slide out of place," "fall into error or fault." Sense of "pass unguarded or untaken" is from mid-15c. That of "slide, glide" is from 1520s. Transitive sense from 1510s; meaning "insert surreptitiously" is from 1680s. Related: Slipped; slipping. To slip up "make a mistake" is from 1855; to slip through the net "evade detection" is from 1902.
mid-15c., "edge of a garment;" 1550s, "narrow strip," probably from Middle Low German or Middle Dutch slippe "cut, slit," possibly related to Old English toslifan "to split, cleave." Sense of "narrow piece of paper" (e.g. pink slip) in 1680s.
in various senses from slip (v.). Meaning "act of slipping" is from 1590s. Meaning "mistake, minor fault, blunder" is from 1610s. Sense of "woman's sleeveless garment" (1761) is from notion of something easily slipped on or off (cf. sleeve). To give (someone) the slip "escape from" is from 1560s. Meaning "landing place for ships" is mid-15c.; more technical sense in ship-building is from 1769. Slip of the tongue (1725) is from earlier slip of the pen (1650s), which makes more sense as an image.
"potter's clay," mid-15c., "mud, slime," from Old English slypa, slyppe "slime, paste, pulp, soft semi-liquid mass," related to slupan "to slip" (see sleeve).
"sprig or twig for planting or grafting, small shoot," late 15c., of uncertain origin. Cf. Middle Dutch slippe, German schlippe, schlipfe "cut, slit, strip." Hence "young person of small build" (1580s, e.g. a slip of a girl); see slip (n.1).
in engineering and physics, sliding displacement along a plane of one part of a crystal relative to the rest of the crystal under the action of shearing forces-that is, forces acting parallel to that plane. Much of the permanent, or plastic, deformation of materials under stress is the result of slip within the individual crystals that constitute the material. Slip and an alternate mode of deformation, twinning, are the only ways that crystals in solids can be permanently deformed. In slip, all the atoms on one side of the slip (or glide) plane do not slide simultaneously from one set of positions to the next. The atoms move sequentially one row at a time into the next position along the plane because of structural defects or spaces, called edge dislocations, in the crystal that move at the same rate in the opposite direction.