verb (handicaps, handicapping, handicapped)[with object]
mid 17th century: from the phrase hand in cap; originally a pastime in which one person claimed an article belonging to another and offered something in exchange, any difference in value being decided by an umpire. All three deposited forfeit money in a cap; the two opponents showed their agreement or disagreement with the valuation by bringing out their hands either full or empty. If both were the same, the umpire took the forfeit money; if not, it went to the person who accepted the valuation. The term handicap race was applied (late 18th century) to a horse race in which an umpire decided the weight to be carried by each horse, the owners showing acceptance or dissent in a similar way: hence in the late 19th century handicap came to mean the extra weight given to the superior horse
See handicapped (usage).
Handicap is an exception to the rule that you only double the last letter when adding -ing or -ed to a word ending in a vowel plus a consonant if the stress is at the end of the word. In this case, the stress is at the beginning of the word, but you should still double the p: (handicaps, handicapping, handicapped).