late 14c., "to fix the mind upon for careful examination, meditate upon," also "view attentively, scrutinize; not to be negligent of," from Old French considerer (13c.) "reflect on, consider, study," from Latin considerare "to look at closely, observe," probably literally "to observe the stars," from assimilated form of com "with, together" (see con-) + sidus (genitive sideris) "heavenly body, star, constellation" (see sidereal).
Perhaps a metaphor from navigation, but more likely reflecting Roman obsession with divination by astrology. Tucker doubts the connection with sidus, however, because it is "quite inapplicable to desiderare," and suggests derivation instead from the PIE root of English side meaning "stretch, extend," and a sense for the full word of "survey on all sides" or "dwell long upon."
From 1530s as "to regard in a particular light." Related: Considered; considering.