testis (n.)

late 13c., in law, "last will, expressing the final disposition of one's property," from Latin testamentum "a last will, publication of a will," from testari "make a will, be witness to," from testis "witness," generally accepted to be from PIE *tri-st-i- "third person standing by," from root *tris- "three" (see three) on the notion of "third person, disinterested witness."

Use of testament 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 diathēkē and kainē diathēkē. But here Late Latin testamentum is a confusion of the two meanings of Greek diathēkē, which meant both "covenant, dispensation" and "will, testament." Testament had been used in the sense of "covenant between God and mankind" in English from early 14c. (as in the account of the Last Supper; see testimony), but the word subsequently was interpreted as Christ's "last will."

1756 as a type of women's tall dressed hair or wig, 1756, from French tête, literally "head," Old French teste "head," from Latin testa, literally "piece of earthenware, tile, potsherd; earthen pot, pitcher, jug; shell of shellfish." In Watkins, etc., this is derived from PIE root *teks- "to weave," also "to fabricate." But de Vaan (2008) writes, "Derivation from the root *tek- 'to build' is unlikely for semantic reasons," and "The word testa is probably a loanword, as words for vessels often are." 

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 Proto-Germanic root of English cup (n.).