digitigrade adj.

"walking on the whole sole of the foot" (opposed to digitigrade), 1831, from French plantigrade "walking on the sole of the foot" (1795), from Latin planta "sole of the foot" (from nasalized form of PIE root *plat- "to spread") + gradi "to walk, go, step" (from PIE root *ghredh- "to walk, go"). Used of man and quadrupeds (bears, etc.) whose heels touch the ground in walking.

late 14c., "numeral below 10," from Latin digitus "finger or toe" (also with secondary meanings relating to counting and numerals), considered to be related to dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly"). The numerical sense is because numerals under 10 were counted on fingers. The "finger or toe" sense in English is attested from 1640s.

Proto-Indo-European root meaning "to walk, go." 

It forms all or part of: aggress; aggression; aggressive; centigrade; congress; degrade; degree; degression; digress; digression; egress; gradation; grade; gradual; graduate; grallatorial; gravigrade; ingredient; ingress; plantigrade; progress; progression; regress; regression; retrograde; retrogress; tardigrade; transgress; transgression.

It is the hypothetical source of/evidence for its existence is provided by: Latin gradus "a step, a pace, gait," figuratively "a step toward something, a degree of something rising by stages;" gradi "to walk, step, go;" Lithuanian gridiju, gridyti "to go, wander;" Old Church Slavonic gredo "to come;" Old Irish in-greinn "he pursues."