The spelling of the phrase "hast oneself" can be explained using IPA phonetic transcription. It is pronounced as /hæst wʌnself/ and is an archaic form of "you have yourself." The word "hast" is the second person singular present tense of "have" and "oneself" is a reflexive pronoun that denotes the importance of self-care. While this phrase has largely fallen out of use, it serves as a reminder of the importance of taking care of oneself.
The phrase "hast oneself" is derived from the Middle English verb "hasten", which originated from the Old French word "haster". In turn, "haster" can be traced back to the Latin word "festinare", meaning "to hurry" or "to hasten". Over time, the phrase "hast oneself" developed as a way of expressing someone's quick or immediate actions. However, in modern English, this construction is quite archaic and is not commonly used in contemporary language.