The compound adjective 'two-tailed' is a transparent modern English formation built from native Germanic elements. The numeral 'two' descends from the Proto-Indo-European root dwóh₁, which is the source for the basic number across many Indo-European languages. The word 'tail' originates from the Proto-Germanic taglaz, referring specifically to a hairy appendage. The suffix '-ed', used to form participial adjectives, was already productive in Old English. The term's literal meaning, "having two tails," is straightforward, but its specialized statistical sense is a 20th-century metaphorical extension. In this context, the "tails" refer to the extreme ends of a probability distribution curve, and a "two-tailed" test examines the possibility of deviation in both positive and negative directions, conceptually mirroring the image of a creature with two distinct appendages.

Structure: two (number) + tail (appendage) + ed (adjective-forming suffix) - two: From Old English 'twā', of Germanic origin; related to Dutch 'twee' and German 'zwei', from an Indo-European root shared by Latin 'duo' and Greek 'duo'. Functions as a numerical quantifier. - tail: From Old English 'tægl', 'tægel', of Germanic origin. Functions as a noun denoting a flexible appendage. - ed: A suffix forming adjectives from nouns, from Old English '-ede', '-ode', '-ad'. Functions as an adjectival suffix meaning "having the characteristics of" or "provided with".