light

The English word light hides two completely separate Old English words that happened to collapse into the same spelling. They have nothing to do with each other in meaning — one is about brightness, the other about weight — but both come from the Germanic stock that predates Latin's arrival in English.

Branch 1: lēoht = brightness, the opposite of dark. This is the light you switch on, the light of the sun. From this single Germanic root grew a whole family of compounds, each one just naming where the light comes from:

- sun + light → sunlight: light from the sun
- moon + light → moonlight: light from the moon
- star + light → starlight, candle + light → candlelight, fire + light → firelight: the same pattern, one source after another
- day + light → daylight: the light of the day itself

When you put a house over a light, you get a lighthouse — a tower whose whole purpose is to shine. When light fades at dusk, you get twilight (twi- is an old word for 'two/half', so it is the 'half-light' between day and night).

This brightness sense also goes figurative. To highlight something is to throw a strong light on it so it stands out — first literally with a marker pen, then metaphorically (highlight a problem). To enlighten someone is to bring light into their mind — en- (to put into) + lighten (make bright) — so ignorance is darkness and knowledge is light. The 18th-century Enlightenment named an entire age after that image: reason flooding light into a dark world.

Branch 2: līht = not heavy, easy to lift. This is the light of a light bag, a light meal. From here come lightly (gently, with little weight or seriousness), lighten (to make less heavy — lighten the load), lightweight (low in weight, also a boxing class, also dismissively 'a person of no substance'), and ultralight (ultra- extremely + light = extremely light).

Notice that lighten sits in both branches: you can lighten a room (make it brighter) or lighten a backpack (make it less heavy). English never bothered to keep the two words apart, so the same form does double duty. The same is true of lightness (brightness or low weight).

The rule of thumb for this family: ask what kind of light? If it answers 'how bright / how lit' → branch 1 (sunlight, lighthouse, enlighten). If it answers 'how heavy' → branch 2 (lightweight, lighten the load, ultralight).