four-door (adj.)

"1 more than three, twice two; the number which is one more than three; a symbol representing this number;" Old English feower "four; four times," from Proto-Germanic *fedwores (source also of Old Saxon fiuwar, Old Frisian fiower, fiuwer, Frankish *fitter-, Dutch vier, Old High German fior, German vier, Old Norse fjorir, Danish fire, Swedish fyra, Gothic fidwor "four"), from PIE root *kwetwer- "four." The phonetic evolution of the Germanic forms has not been fully explained; Watkins explains the -f- as being from the following number (Modern English five).

To be on all fours is from 1719; earlier on all four (14c.). Four-letter word as a euphemism for one of the short words generally regarded as offensive or objectionable is attested from 1923; four-letter man is recorded from 1920 (apparently as a euphemism for a shit). Compare Latin homo trium litterarum, literally "three-letter man," a euphemism for fur "a thief." A four-in-hand (1793) was a carriage drawn by four horses driven by one person; in the sense of "loosely tied necktie" it is attested from 1892. To study The History of the Four Kings (1760, compare French Livres des Quatre Rois) contains an old euphemistic slang phrase for "a pack of cards," from the time when card-playing was considered a wicked pastime for students. Slang 4-1-1 "essential information" (by 1993) is from the telephone number called to get customer information. The four-color problem so called from 1879. The four-minute mile was attained 1954.

"movable barrier, commonly on hinges, for closing a passage into a building, room, or other enclosure," c. 1200, a Middle English merger of two Old English words, both with the general sense of "door, gate": dor (neuter; plural doru) "large door, gate," and duru (fem., plural dura) "door, gate, wicket." The difference (no longer felt in Old English) was that the former came from a singular form, the latter from a plural.

Both are from Proto-Germanic *dur-, plural *dures (source also of Old Saxon duru, Old Norse dyrr, Danish dr, Old Frisian dure, dore, dure, Old High German turi, German Tür). This is from PIE root *dhwer- "door, doorway."

Middle English had both dure and dor; the form dore predominated by 16c. but was supplanted later by door. The oldest forms of the word in IE languages frequently are dual or plural, leading to speculation that houses of the original Indo-Europeans had doors with two swinging halves.

Figurative sense of "means of opportunity or facility for" was in Old English. Phrase door to door "house to house" is from c. 1300; as an adjective, in reference to sales, by 1902.

A door is what a dog is perpetually on the wrong side of. [Ogden Nash]