early 13c., "of or pertaining to the head," from Old French capital, from Latin capitalis "of the head," hence "capital, chief, first," from caput (genitive capitis) "head" (from PIE root *kaput- "head"). Meaning "main, principal, chief, dominant, first in importance" is from early 15c. in English. The modern informal sense of "excellent, first-rate" is dated from 1762 in OED (as an exclamation of approval, OED's first example is 1875), perhaps from earlier use of the word in reference to ships, "first-rate, powerful enough to be in the line of battle," attested from 1650s, fallen into disuse after 1918. Related: Capitally.
A capital letter "upper-case latter," of larger face and differing more or less in form (late 14c.) is so called because it stands at the "head" of a sentence or word. Capital gain is recorded from 1921. Capital goods is recorded from 1899.
A capital crime or offense (1520s) is one that involves the penalty of death and thus affects the life or "head" (capital had a sense of "deadly, mortal" from late 14c. in English, as it did earlier in Latin). The felt connection between "head" and "life, mortality" also existed in Old English: as in heafodgilt "deadly sin, capital offense," heafdes þolian "to forfeit life." Capital punishment was in Blackstone (1765) and classical Latin capitis poena.