Shikantaza (只管打坐?) is a Japanese translation of a Chinese term for zazen introduced by Rujing, a monk of the Caodong school of Zen Buddhism. In Japan, it is associated with the Soto school.
The term shikantaza is attributed to Dōgen's teacher Tiantong Rujing (1162-1228), and it literally means, "nothing but () precisely () sitting ()." In other words, Dōgen means, "doing only zazen whole-heartedly" or "single-minded sitting."
Shikantaza is the Sino-Japanese reading of the Chinese words zhị̌guǎn 只管 "by all means; merely, simply; only concerned with" and dǎzuò 打坐 "[Buddhism/Daoism] sit in meditation". The Digital Dictionary of Buddhism translates shikan or zhǐguǎn 只管 as "to focus exclusively on", taza or dǎzuò 打坐 as "to squat, sit down cross-legged", which corresponds with Sanskrit utkuṭuka-stha, and translates shikan taza from zhǐguǎn dǎzuò 只管打坐 (or qíguǎn dǎzuò 祇管打坐 with qí "earth god; local god") as "meditation of just sitting", explained as the "Zen form of meditation chiefly associated with the Sōtō school, which places emphasis on emptying the mind, in contrast to the kōan method".
James Ishmael Ford says some authors hypothetically trace the root of shikantaza "just sitting" to vipassana meditation, but "this is far from certain." Japanese has many homophones pronounced shikan, and this etymological mix-up about shikan 只管 "only; just" stems from a more commonly used word that translates the Sanskrit "śamatha and vipaśyanā," names for the two basic forms of Buddhist meditation: Japanese shikan 止観 "concentration and observation" (as practiced by the Tendai sect), from Chinese zhǐguān 止觀 "[Buddhism] keep mental calm while observing the universe" (cf. the Mohe Zhiguan), which compounds shi or zhǐ 止 "stop; stabilize; śamatha" and kan or guān 觀 "observe; contemplate; vipaśyanā". An instance of the confusion of 止観 for 只管 is Steve Hagen's claim that "shi [Hagen is referring to Dōgen's '只'] means tranquility [= '止'], kan [Hagen is referring to Dōgen's '管'] means awareness [= '観'], ta means hitting exactly the right spot (not one atom off), and za means to sit."