'anaesthatic' is a variant spelling of 'anaesthetic', which originates from the Greek word 'anaisthēsia', where 'an-' meant 'without' and 'aisthēsis' meant 'sensation'.

'anaisthēsia' was adopted into modern English as 'anaesthetic', and 'anaesthatic' appeared as a rare variant spelling.

Initially, it meant 'without sensation', and over time it came to refer specifically to substances or effects that cause loss of sensation, especially in medical contexts.