Other forms: lightened, lightening, lightens
To lighten something is to make it less heavy, or to make it less dark. Taking two books out of a rucksack lightens it; a drop of white paint lightens the grey; the sky lightens before the sun comes up.
Both meanings are alive and the sentence around it tells you which. Weight, real and figurative: lighten the load, lighten somebody's workload, lighten the tax burden on families. Colour and light: a summer lightens your hair, and dawn lightens the sky. A third use is about mood — a joke to lighten the atmosphere in a tense room — and lighten up is what you say to somebody taking things too seriously, which lands badly more than half the time. The opposites split too: darken for colour, and for weight English reaches for weigh down.
1verblightenTo make something less heavy, or to become less heavy.
“They lightened the boat by throwing the crates overboard.”
2verbbrighterTo make something brighter or paler, or to become brighter or paler.
“Lemon juice lightens your hair in the sun.”
3verbmoodTo make a mood or a situation less serious or less hard.
“He told a bad joke to lighten the mood.”
Lighten is Middle English lightnen — the word light with -en fixed on the end, the same -en that turns hard into harden and soft into soften. That ending is a small machine for making verbs out of adjectives, and English has run light through it and kept the results for both senses.
Middle English
lightnen
English, today
lighten
From Middle English lightnen, equivalent to light + en.
Etymology from Wiktionary, CC BY-SA