Origins
The English verb 'adhere' presents a satisfying case of a word whose physical and metaphorical meanings developed in tandem, both flowing naturally from its Latin source. To stick to a surface and to stick to a principle are, etymologically, the same act.
The word enters English in the 1590s, borrowed from French 'adhérer' or directly from Latin 'adhaerēre,' meaning 'to stick to' or 'to cling to.' The Latin verb is a compound of 'ad-' (to, toward) and 'haerēre' (to stick, to be fixed, to cling). The origin of 'haerēre' itself is disputed. Some etymologists connect it to a PIE root *ghais- meaning 'to stick,' but this reconstruction is not universally accepted. What is certain is that 'haerēre' was thoroughly established in classical Latin with a wide range of both literal and figurative uses.
Cicero used 'adhaerēre' both physically (things sticking together) and abstractly (people clinging to opinions or factions). This dual usage passed intact into English. The physical sense — glue adheres, a bandage adheres, paint adheres to a wall — arrived first and remains standard. The abstract sense — adhering to principles, adhering to a schedule, adhering to a treaty — developed quickly in the early seventeenth century and has become equally common.
Latin Roots
The family of English words descended from Latin 'haerēre' is extensive and somewhat surprising. The most obvious relatives are 'adherent' (one who sticks to a cause), 'adhesion' (the act of sticking), and 'adhesive' (something that causes sticking). Less obvious but equally direct is 'cohere' (from 'co-' + 'haerēre,' to stick together), along with 'coherent,' 'cohesion,' and 'cohesive.' The prefix 'in-' (meaning 'in' rather than 'not') produced 'inhaerēre' (to stick in), source of 'inhere' and the crucial adjective 'inherent' — a quality that 'sticks in' something, an inseparable attribute.
Perhaps most surprisingly, 'hesitate' also descends from 'haerēre.' The Latin frequentative 'haesitāre' meant 'to stick repeatedly' or 'to keep getting stuck,' and from this developed the sense of faltering in speech or being unable to proceed — the modern meaning of hesitation. The semantic link is vivid: a hesitant speaker is one whose words stick in the throat, who gets stuck between alternatives. The connection between adhesion and hesitation, invisible on the surface of modern English, is perfectly transparent in Latin.
The adjective 'adhesive' took on particular technical significance in the modern era. The development of synthetic adhesives in the twentieth century — from Scotch tape (1930) to superglue (cyanoacrylate, discovered accidentally in 1942) — made 'adhesive' a ubiquitous term in manufacturing, medicine, and daily life. The medical 'adhesive bandage' (Band-Aid, trademarked 1920) brought the Latin root into millions of homes.
Cultural Impact
In the Romance languages, the descendants of 'adhaerēre' are regular: French 'adhérer,' Spanish 'adherir,' Portuguese 'aderir,' Italian 'aderire.' Each maintains both the physical and figurative senses. German, lacking a native cognate, uses 'anhaften' (to stick on) for the physical sense and 'sich halten an' (to hold to) for the figurative, though the borrowed 'adhärieren' exists in technical contexts.
The word 'adhere' thus offers an elegant demonstration of how metaphorical extension works across centuries. The physical experience of things sticking together is one of humanity's most basic observations, and the leap from physical adhesion to loyalty, commitment, or obedience is so natural that it occurred independently in Latin and was immediately understood when transplanted into English. To adhere is to stick — whether the sticking is done with glue or with conviction.