portsider n.

"the left side of a ship" (looking forward from the stern),  1540s, probably from the notion of "the side facing the harbor" (when a ship is docked); thus from port (n.1). On old-style vessels the steering oar was on the right side, thus they would tie up at a wharf on the other side. It replaced larboard in common usage to avoid confusion with starboard; officially so by Admiralty order of 1844 and U.S. Navy Department notice of 1846. As an adjective by 1857.

U. S. Navy Department, Washington, Feb. 18, 1846.

It having been repeatedly represented to the Department that confusion arises from the use of the words "larboard" and "starboard"' in consequence of their similarity of sound, the word "port" is hereafter to be substituted for "larboard." George Bancroft, Sec. of the Navy.

The whalemen are the only of seamen who have not adopted the term port instead of larboard, except in working ship. The larboard boat was this boat to their great-grandfathers, and it is so with the present generation. More especially is this the case in the Atlantic and South Pacific fleets; but recently the term port-boat has come into use in the Arctic fleet. [Fisheries of U.S., V. ii. 243, 1887]

Old English side "flanks of a person, the long part or aspect of anything," from Proto-Germanic *sīdō (source also of Old Saxon sida, Old Norse siða, "flank; side (of meat); coast," Danish side, Swedish sida, Middle Dutch side, Dutch zidje, Old High German sita, German Seite), from adjective *sithas "long" (source of Old English sid "long, broad, spacious," Old Norse siðr "long, hanging down"), from PIE root *se- "long; late" (see soiree).

The "long part of anything" sense is preserved hillside, it also was in 16c.-17c. side-coat "long coat." From 14c. as "lateral half of the body of a slaughtered animal." In reference to bacon, it indicates position relative to the ribs. The meaning "a region, district" is from c. 1400, as in South Side, countryside.

The figurative sense of "position or attitude of a person or set of persons in relation to another" (as in choose sides, side of the story) is recorded by mid-13c. As "an aspect" of anything immaterial (the bright side, etc.), by mid-15c.

The meaning "one of the parties in a transaction" is from late 14c.. The sense of "one of the parties in a sporting contest or game" is from 1690s. The meaning "music on one side of a phonograph record" is attested by 1936. As short for side-dish, by 1848.

The phrase side by side "close together and abreast, placed with sides near together" is recorded from c. 1200. Colloquial on the side "in addition," especially "unacknowledged," with connotations of "illicit, shady," is by 1893.