testis n.

late 13c., "last will disposing of property," from Latin testamentum "a last will, publication of a will," from testari "make a will, be witness to," from testis "witness," from PIE *tri-st-i- "third person standing by," from root *tris- "three" (see three) on the notion of "third person, disinterested witness."

Use in reference to the two divisions of the Bible (early 14c.) is from Late Latin vetus testamentum and novum testamentum, loan-translations of Greek palaia diatheke and kaine diatheke. Late Latin testamentum in this case was a confusion of the two meanings of Greek diatheke, which meant both "covenant, dispensation" and "will, testament," and was used in the former sense in the account of the Last Supper (see testimony) but subsequently was interpreted as Christ's "last will."

as a type of women's tall dressed hair or wig, 1756, from French tête "head," Old French teste, from Latin testa, literally "piece of earthenware, tile, potsherd; earthen pot, pitcher, jug; shell of shellfish," related to Latin testudo "tortoise" and texere "to weave" (compare Lithuanian tištas "vessel made of willow twigs"), from PIE root *teks- "to weave," also "to fabricate."

The "head" sense arose in Vulgar Latin, perhaps as a humorous use of the "jug, pot" meaning, or via Late Latin use of testa as "skull," from testa (capitis) "shell (of the head)." Compare German Kopf "head" from the root of English cup (n.).