Origins
The English word 'incur' entered the language in the early fifteenth century, from Latin 'incurrere' (to run into, to rush upon, to attack). The Latin verb combines 'in-' (into, upon) and 'currere' (to run), creating the image of running into something — charging forward into contact, collision, or confrontation.
In classical Latin, 'incurrere' was a vivid physical verb. Soldiers incurred — ran into — the enemy line. A ship incurred rocks. A traveler incurred bandits. The word carried implications of speed, force, and often danger. The construction 'incurrere in + accusative' (to run into something) was the standard form, and the something run into was typically unpleasant: danger, punishment, blame, hatred.
English narrowed the word significantly. The physical sense of running or charging disappeared almost entirely. What remained was the abstract sense of encountering negative consequences as a result of one's own actions. To incur a debt is to run into an obligation. To incur a penalty is to run into a punishment. To incur someone's wrath is to run into their anger. The word almost always takes a negative object: one incurs costs, losses, debts, penalties, risks, displeasure, criticism, or liability. One does not normally 'incur' a benefit or a reward (though this is not strictly ungrammatical, it sounds odd to English ears).
French Influence
The negative coloring of 'incur' is not inherent in the Latin — 'incurrere' could describe running into friends as easily as enemies. The narrowing happened gradually in Late Latin and Old French, as the word became increasingly associated with running into trouble. By the time it reached English, the association with negative consequences was nearly fixed.
The military noun 'incursion' (from Latin 'incursio,' a running in, an assault) preserves the original physical sense of 'incurrere' more faithfully than the verb 'incur.' An incursion is a sudden, forceful entry into enemy territory — an army running into a foreign land. The word implies brevity and limited scope: an incursion is smaller and shorter than an invasion. Modern usage extends it to non-military contexts: an incursion into someone's privacy, an incursion into a competitor's market share.
The relationship between 'incur' and 'incursion' illustrates how the same Latin root can produce English words with quite different feels. 'Incur' is abstract, formal, and focused on consequences. 'Incursion' is concrete, dynamic, and focused on action. Both descend from 'incurrere' (to run into), but they split the parent verb's meaning: 'incur' took the self-inflicted consequences, 'incursion' took the military charge.
Later History
In legal usage, 'incur' is particularly important. Liabilities are incurred, obligations are incurred, penalties are incurred. The word implies agency — the person who incurs did something (or failed to do something) that caused the consequence. This implication of responsibility makes 'incur' a key verb in contracts, statutes, and judicial opinions.
In financial usage, 'incur' is the standard verb for taking on costs and debts. 'Incurred expenses,' 'incurred losses,' 'incurred but not reported (IBNR) claims' — these are fundamental terms in accounting, insurance, and business reporting. The word's implication of voluntariness (you incurred the expense through your actions) distinguishes it from 'suffer' (which implies passivity) and 'accrue' (which implies automatic accumulation).
Within the 'currere' family, 'incur' occupies the position of running into: direct confrontation with consequences. Where 'occur' is events running toward you (happening), 'recur' is events running back (repeating), and 'concur' is running together (agreeing), 'incur' is running into — headlong collision with the results of one's own choices.