1772, "relating to or of the nature of custody or guardianship," from custody (Latin custodia) + -al (1). It is in French by 1752.
Entries linking to custodial
custody n.
mid-15c., "a keeping, a guarding, safe-keeping, protection, defense," from Latin custodia "guarding, watching, keeping," also "prison," from custos (genitive custodis) "guardian, keeper, protector," from PIE root *(s)keu- "to cover, conceal." Meaning "restraint of liberty, confinement" is from 1580s.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).