thin (adj.)

Old English þynne "narrow, lean, scanty, not dense; fluid, tenuous; weak, poor," from Proto-Germanic *thunni "thin" (source also of West Frisian ten, Middle Low German dunne, Middle Dutch dunne, Dutch dun, Old High German dunni, German dünn, Old Norse þunnr, Swedish tunn, Danish tynd), from PIE *tnu- "stretched, stretched out" (hence "thin"), from root *ten- "to stretch" (source also of Latin tenuis "thin, slender").

These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre. [Shakespeare, "The Tempest," IV.i.150, 1610]

"Loose or sparse," hence "easily seen through," with figurative extensions. Related: Thinly; thinness. Thin-skinned is attested from 1590s; the figurative sense of "touchy" is from 1670s.

thin (v.)

Old English þynnian "to make thin, lessen, dilute," also intransitive, "become thin," from thin (adj.). Intransitive sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Compare similarly formed German dünnen, Dutch dunnen. Related: Thinned; thinning.

Definitions of thin
thin (adj.)
of relatively small extent from one surface to the opposite or in cross section;
a thin layer of paint
a thin chiffon blouse
thin wire
a thin book
thin (adj.)
lacking excess flesh; "Yon Cassius has a lean and hungry look"-Shakespeare;
you can't be too rich or too thin
Synonyms: lean
thin (adj.)
very narrow;
a thin line across the page
Synonyms: slender
thin (adj.)
not dense;
a thin beard
Synonyms: sparse
thin (adj.)
relatively thin in consistency or low in density; not viscous;
thin oil
skimmed milk is much thinner than whole milk
air is thin at high altitudes
a thin soup
thin (adj.)
(of sound) lacking resonance or volume;
a thin feeble cry
thin (adj.)
lacking spirit or sincere effort;
a thin smile
thin (adj.)
lacking substance or significance;
a thin plot
Synonyms: flimsy / fragile / slight / tenuous
thin (v.)
lose thickness; become thin or thinner;
thin (v.)
make thin or thinner;
thin (v.)
lessen the strength or flavor of a solution or mixture;
Synonyms: dilute / thin out / reduce / cut
thin (v.)
take off weight;
Synonyms: reduce / melt off / slim / slenderize / slim down
thin (adv.)
without viscosity;
the blood was flowing thin
Synonyms: thinly