Frequent started as a crowd, not a clock. Latin frequēns meant 'crowded, packed, numerous' — it described places full of people, not events repeated in time. A frequēns senātus was a well-attended senate. A frequēns via was a busy road.
The temporal meaning grew from a spatial metaphor. Events that happen often are, in a sense, crowded together in time. Just as people fill a room, occurrences fill a schedule. By the time English borrowed the word through Old French in the 15th century, both meanings were active.
English preserves the older, spatial sense in the verb form. To frequent a place is to visit it regularly — to be one of the crowd that fills it. A frequenter of cafés is someone whose presence makes the café crowded. The noun frequency works both ways: the frequency of earthquakes (temporal) and radio frequency (spatial, in the sense of wave density).
The deeper etymology remains debated. Many scholars connect frequēns to a PIE root *bʰrek- meaning 'to cram' or 'to stuff'. If correct, the original image is of things pressed tightly together — whether people in a marketplace or events in a calendar.
Infrequent, the negation, arrived later in the 16th century. It means uncrowded in time — events spread so far apart they no longer press against each other.