后綴-ac來源于法語-aque、拉丁語-acus或希臘語-akos,屬于形容詞或名詞后綴，意思是“有...的，患…的，具有…特征或特點的，關于”，有同類后綴：-acal。把后綴-iac也放在這里，是因為這二個后綴有很大的關系，如果您稍微會點英語，請讀一下，下面的純英文關于這個后綴-ac來源，會了解這二個后綴關系及其來源。

英文詞源：All words ending in -ac from this source include an i from the original Latin or Greek root, so making it in effect -iac.

Some are adjectives: cardiac (Greek kardia, heart), of the heart; iliac (Latin ilia, entrails), relating to the ilium, a bone in the pelvis, or to the nearby regions of the lower body. Others are nouns: aphrodisiac, a food, drink, or drug that stimulates sexual desire; zodiac, the belt of the heavens that includes the apparent paths of the sun, moon, and principal planets. Some can be both noun and adjective: amnesiac, hypochondriac, insomniac.

Nouns often have linked adjectives in -al (see -al), as in zodiacal and ammoniacal. In a few cases, adjectives in both forms co-exist, as with paradisiac and paradisiacal, or heliac and heliacal (though in both these cases the form in -acal is much more common).

Some words in -ac have related forms in -ic: demoniac and demonic, haemophiliac and haemophilic, maniac and manic. In each of these cases the form in -ic is an adjective, irrespective of the role of the form in -ac. An exception is amnesiac, in which amnesic can be both adjective and noun.

同綴詞：

1.maniac [man = mania 瘋狂 + -iac(或-i- 中綴 + -ac) 形容后綴 →]
adj.瘋狂的,瘋狂的人

2.cardiac [card 心，來源于希臘文kardia,同上]
adj. 心臟的

3.heliac [hel 太陽 ,同上]
adj.太陽的

4.coeliac [coel ，同上]
adj.腹腔的,下腹的

5.demoniac [demon 惡魔 ,同上]
adj.惡魔的；兇惡的

6.insominac [in- 不 + som 睡眠 同上 →]
adj. 失眠的;令人失眠的

7.theomaniac [the神 -o-連接字母 ,同上]
n.宗教狂人