大毘盧遮那成佛經疏 13.78

13.78

如來者即是如實也。猶如來舌常作如語、不誑語、不異語，以如是真實，故常住也。

'Tathāgata' means 'thusness' (yathābhūta). Just as the Tathāgata's tongue always speaks words of thusness, non-deceptive words, non-differing words, because of such truthfulness, it is eternal.