'arccos' originates from modern mathematical English, formed as an abbreviation of 'arc cosine'. 'arc' comes from Latin 'arcus' meaning 'arch', and 'cos' is short for 'cosine', which itself comes from Medieval Latin 'cosinus'.

'cosine' changed from Medieval Latin 'cosinus' (used in early trigonometry) and entered English mathematical vocabulary; 'arc cosine' was coined to denote the angle (arc) corresponding to a given cosine value, and that phrase was later contracted to the shortened written form 'arccos'.

Initially, 'arc cosine' described the arc (angle) corresponding to a cosine value; over time it came to name the inverse function of cosine and is now commonly written and spoken as 'arccos'.