From Latin 'intactus' (untouched), from 'tangere' (to touch) — what has not been touched remains whole.
From Latin intactus (untouched, uninjured, undefiled), from in- (not) + tactus, past participle of tangere (to touch). Tangere derives from PIE *teg- (to touch, to handle), attested in Welsh taro (to strike) and Old Irish do·tét (approaches). The same root underlies tangent (touching a curve at one point), tangible (capable of being touched), contaminate (to touch together, to defile by contact), contagion (a touching together, a spreading by contact), and contact (a touching with). Something intact has literally not been touched by whatever damage, interference, or alteration is implied. The word has been used in English since the 15th century, first of physical objects and later
The medical term 'intact skin' — skin that has no breaks, cuts, or lesions — preserves the exact original Latin meaning: skin that has not been touched (i.e., not violated or broken). Intact skin is the body's first and most important barrier against infection, which etymologically means that what has 'not been touched' protects you from 'contagion' (touching-together) — the root *tag- defending against itself.