"of the nature of a paradox," 1580s, from paradox + -ical. Meaning "inconsistent with itself" is by 1630s. Competing forms were paradoxal (1560s), paradoxial (1620s), but these survive in niches, if at all. Related: Paradoxically.
1530s, "a statement contrary to common belief or expectation," from French paradoxe (14c.) and directly from Latin paradoxum "paradox, statement seemingly absurd yet really true," from Greek paradoxon "incredible statement or opinion," noun use of neuter of adjective paradoxos "contrary to expectation, incredible," from para- "contrary to" (see para- (1)) + doxa "opinion," from dokein "to appear, seem, think" (from PIE root *dek- "to take, accept").
Originally with notions of "absurd, fantastic." Meaning "statement that is seemingly self-contradictory yet not illogical or obviously untrue" is from 1560s. Specifically in logic, "a statement or proposition from an acceptable premise and following sound reasoning that yet leads to an illogical conclusion," by 1903.
compound adjectival word-forming element, usually interchangeable with -ic but sometimes with specialized sense (such as historic/historical, politic/political), Middle English, from Late Latin -icalis, from Latin -icus + -alis (see -al (1)). Probably it was needed because the forms in -ic often took on a noun sense (for example physic). Forms in -ical tend to be attested earlier in English than their twins in -ic.
<a href="https://www.etymonline.com/word/paradoxical">Etymology of paradoxical by etymonline</a>
Harper, D. (n.d.). Etymology of paradoxical. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/paradoxical