King of Gath in the time of David and Solomon (I Sam. xxi.-xxix. 1; I Kings, ii.). David, when fleeing from Saul, twice sought asylum with Achish, the first time incognito. He was, however, recognized, whereupon he feigned madness, and escaped (I Sam. xxi. 10-15, xxii. 1). The second time he was also recognized, but was well treated as a supposed enemy of Saul (I Sam. xxvii.). Achish led the Philistine attack on Israel which resulted in the death of Saul and his sons. He was also at the battle of Gilboa (I Sam. xxviii.-xxxi.). Two servants of Shimei fled to Achish (I Kings, ii. 39-46). The superscription to Ps. xxxiv. reads "Abimelech," apparently by error for Achish.
The Haggadah elaborates David's insanity as follows: Among Achish's body-guard were the brothers of Goliath, who immediately sought to slay their brother's conqueror. Achish forbade this, pointing out that the combat had been a fair one. The brothers retorted that then, according to the terms of the agreement (I Sam. xvii 9), Achish must relinquish his throne to David. The only way out of this complication was for David to feign madness, but just at that time a daughter of Achish became really insane, and her mania was augmented by David's actions; therefore he was driven away (Midr. Teh. xxxiv.).