Origins
The word 'constant' entered English around 1380 from Old French 'constant,' which derives from Latin 'cōnstāns' (standing firm, steady, unchanging), the present participle of the verb 'cōnstāre' (to stand together, to stand firm, to be consistent, to agree). The Latin verb is a compound of 'com-' (together, or functioning as an intensifier) and 'stāre' (to stand), from PIE *steh₂- (to stand). The original image is of something that stands firmly, does not waver or fall — a physical metaphor that was quickly extended to moral steadfastness, logical consistency, and unchanging nature.
The PIE root *steh₂- (to stand) is one of the most productive roots in the Indo-European family, and its descendants dominate the English vocabulary of stability, governance, and existence. From Latin 'stāre' alone, English has inherited dozens of words: 'state' (a condition in which something stands), 'station' (a place where one stands), 'statue' (something that stands), 'stable' (standing firm), 'substance' (that which stands under), 'circumstance' (that which stands around), 'instant' (standing upon — i.e., pressing, urgent), 'distant' (standing apart), 'obstacle' (that which stands in the way), 'contrast' (standing against), 'prostitute' (one who stands forward, i.e., publicly), and 'superstition' (standing over — originally the state of being 'left standing' in awe or fear).
Through Germanic, the same root produced English 'stand,' 'stead' (as in 'instead' and 'homestead'), 'steady,' and 'stud' (a post or prop). Through Greek 'histánai' (to make stand, to set up), it gave English 'stasis' (standing still), 'ecstasy' (standing outside oneself), 'system' (things set together), and the suffix '-stat' (thermostat, rheostat — devices that make things stand still).
French Influence
The commercial sense of 'cōnstāre' deserves special attention. In Latin, the verb developed the meaning 'to be settled at a price, to cost' — the idea being that a price is something that 'stands firm' in the marketplace. This sense produced the Latin noun 'cōnstāre' (used impersonally as 'it costs') and eventually Old French 'coster,' whence English 'cost.' So 'constant' and 'cost' are siblings: both come from 'cōnstāre,' one preserving the sense of 'standing firm' and the other the sense of 'standing at a price.'
In English, 'constant' has operated in three main semantic domains. The first is moral and emotional: a constant friend, constant love, constancy of devotion. This is the primary sense in Shakespeare, who used the word frequently. In 'Julius Caesar,' Brutus says of himself 'I am constant as the northern star,' meaning unwavering in his principles — a declaration that proves tragically ironic. The second domain is scientific and mathematical: a constant in physics (the speed of light, the gravitational constant) or in mathematics (pi, e) is a value that does not change. This usage became prominent in the 17th century with the rise of mathematical physics. The third is everyday: constant noise, constant rain, constant interruption — here 'constant' means 'continuing without pause,' a slight shift from the original sense of 'unchanging' to 'unceasing.'
The abstract noun 'constancy' and the adverb 'constantly' followed the adjective into English. 'Constancy' has retained its moral weight more fully than 'constantly,' which has drifted toward the everyday sense of 'all the time' and lost much of its connection to steadfastness. When someone says 'she is constantly late,' they do not mean she is steadfastly committed to lateness — but the etymological ghost of moral firmness haunts the word.
Latin Roots
The word 'inconstant' (from Latin 'incōnstāns') entered English in the 15th century and served as a powerful term of reproach, especially in matters of love. Shakespeare's Romeo and Juliet, Donne's love poetry, and countless other Renaissance works play on the opposition between constancy and inconstancy as fundamental categories of human character. The idea that a person's worth could be measured by their constancy — their ability to 'stand firm' — reflects the Latin and medieval conviction that moral stability was the highest virtue and moral fickleness the deepest vice.