A small beam, either a tiny shaft of light or a small structural timber.
From 'beam' plus the diminutive suffix '-let' (from Old French, meaning 'small'), following the English pattern of creating small versions of things (like 'booklet' or 'hamlet').
The '-let' suffix is like having a special shrinking tool in English: booklet, droplet, rivulet, piglet. It comes from French but became so natural in English that we use it constantly without thinking about where it came from.
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