means A timekeeping device made of two glass bulbs joined at a narrow neck, measuring a fixed interval by the steady trickle of sand from the upper bulb to the lower one.

from A plain English compound — "hour" plus "glass" — naming exactly what it is: a glass that measures an hour. "Hour" came through Old French from Latin hora and ultimately Greek hōra, meaning a season or time of day. "Glass" is good old Germanic stock. The device itself is medieval, though sand-and-glass timers were prized aboard ships, where a swinging pendulum was useless — sand doesn't care how the sea pitches. The famous female silhouette called "hourglass" is a much later borrowing, the body likened to the device's pinched waist.