Origins
The word 'leg' is one of the most dramatic lexical replacements in the history of English. It is not a native English word at all but a borrowing from Old Norse 'leggr' (leg, limb, bone, hollow stalk), which entered English during the period of intense Norse-English contact in the Danelaw region of England (roughly the ninth to eleventh centuries). The native Old English word for the lower limb was 'sceanca' (shank), which survives in Modern English only as the more restricted term 'shank' — the part of the leg between the knee and the ankle.
The replacement of a core body-part term by a foreign borrowing is extremely unusual in any language. Body parts, kinship terms, and basic verbs are among the most resistant vocabulary items to replacement, because they are learned early, used constantly, and deeply embedded in the mental lexicon. The fact that 'leg' displaced 'shank' testifies to the extraordinary depth of Norse influence on English — this was not a superficial borrowing of exotic vocabulary but a wholesale replacement at the most intimate level of the language, among people who lived together, intermarried, and raised bilingual children.
Old Norse 'leggr' comes from Proto-Germanic *lagjaz, but the further etymology is uncertain. Some scholars have proposed a connection to a PIE root meaning 'limb' or 'to be slack, to bend,' but no consensus has been reached. The word's cognates are confined to the North Germanic languages: Swedish 'lägg' (calf of the leg), Danish 'læg' (calf of the leg), Icelandic 'leggur' (leg). It did not exist in West Germanic before the Norse contact period — Old English, Old High German, and Old Saxon all used other words for the leg.
Modern Usage
The semantic range of Old Norse 'leggr' was broader than that of Modern English 'leg.' It could refer to the leg as a whole, to a single bone (especially a limb bone), or to any hollow stalk or tube. This broader sense survives in some Scandinavian dialects and in compound forms.
Other Norse body-part borrowings in English include 'skin' (Old Norse 'skinn,' replacing native 'hȳd' which survives as 'hide'), 'skull' (probably from Old Norse, though the exact source is debated), and 'neck' (possibly reinforced by Old Norse 'hnakki,' though the Old English form 'hnecca' already existed). But 'leg' remains the most striking example because 'shank' was so thoroughly displaced.
The figurative uses of 'leg' are extensive: 'a leg of a journey' (one segment), 'a table leg,' 'leg up' (an advantage), 'break a leg' (theatrical good-luck wish, of uncertain origin, possibly from the German expression 'Hals- und Beinbruch'). 'Leggings' dates from the eighteenth century. The idiom 'on its last legs' (near collapse) dates from the sixteenth century.