adjective, thin⋅ner, thin⋅nest, adverb, verb, thinned, thin⋅ning.| 1. | having relatively little extent from one surface or side to the opposite; not thick: thin ice. |
| 2. | of small cross section in comparison with the length; slender: a thin wire. |
| 3. | having little flesh; spare; lean: a thin man. |
| 4. | composed of or containing objects, particles, etc., widely separated; sparse: thin vegetation. |
| 5. | scant; not abundant or plentiful. |
| 6. | of relatively slight consistency or viscosity: thin soup. |
| 7. | rarefied, as air. |
| 8. | without solidity or substance; flimsy: a very thin plot for such a long book. |
| 9. | lacking fullness or volume; weak and shrill: a thin voice. |
| 10. | without force or a sincere effort: a thin smile. |
| 11. | lacking body, richness, or strength: a thin wine. |
| 12. | lacking in chroma; of light tint. |
| 13. | Photography. (of a developed negative) lacking in density or contrast through underdevelopment or underexposure. |
| 14. | in a thin manner. |
| 15. | sparsely; not densely. |
| 16. | so as to produce something thin: Slice the ham thin. |
| 17. | to make thin or thinner (often fol. by down, out, etc.). |
| 18. | to become thin or thinner; become reduced or diminished (often fol. by down, out, off, etc.): The crowd is thinning out. |

"These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre." [Shakespeare, "The Tempest," IV.i.150, 1610]The verb is from O.E. þynnian "to make thin" (cf. Ger. dünnen, Du. dunnen); intrans. sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Thin-skinned is attested from 1598; the fig. sense of "touchy" is from 1680.
thin
In addition to the idioms beginning with thin, also see into thin air; on thin ice; spread oneself too thin; through thick and thin; wear thin.