kapdoloro

Kapo plus doloro, head plus pain, and that is the whole word. Notice what happened to the -o of kapo: it dropped. Esperanto lets you keep the connecting vowel or leave it out, and the rule is only that the result must be pronounceable, so kapdoloro is standard and kapodoloro is also correct but clumsier. Keep it when the consonants would pile up; drop it when they would not. More in compounding.

The pattern is fully productive, and this is the reason to learn it as a pattern rather than as a word. Body part plus doloro gives you the whole medical waiting room: dentodoloro toothache, ventrodoloro stomach ache, gorĝdoloro a sore throat, dorsdoloro backache. English makes you learn "toothache" but "sore throat" and "backache", each by rote. Here you learn one join and get the lot. Note also the preposition: you suffer pro kapdoloro, because of a headache, rather than "from" one.