名詞 lunatic:

1. an insane person
   同義詞：madman, maniac
2. a reckless impetuous irresponsible person
   同義詞：daredevil, madcap, hothead, swashbuckler, harum-scarum

形容詞 lunatic:

1. insane and believed to be affected by the phases of the moon
   同義詞：moonstruck

1. I really don't know what kind of man he really is; on one occasion, he is normal, and on another, he acts like a lunatic.
   我真不了解他到底是怎樣的人; 一會(huì)兒正常，一會(huì)兒又像個(gè)瘋子。
2. You're driving on the wrong side of the road, you lunatic!
   你現(xiàn)在是逆行開車，你這個(gè)蠢材！
3. The lunatic fringe are ignored by most members of the party.
   絕大部分黨員對(duì)極端分子不予理睬。

lu.na.tic

adj.（形容詞）
Suffering from lunacy; insane.
精神錯(cuò)亂的：遭受精神錯(cuò)亂的痛苦的；不清醒的
Of or for the insane.
精神病人的
Wildly or giddily foolish:
極端愚蠢的：狂亂地愚蠢的或驚人地愚蠢的：
例句：
a lunatic decision.
一個(gè)極其愚蠢的錯(cuò)誤

Characterized by lunacy or eccentricity.
精神錯(cuò)亂者的或?yàn)榫皴e(cuò)亂病人而設(shè)立的

來源：
Middle English lunatik
中古英語 lunatik
from Old French lunatique
源自 古法語 lunatique
from Latin lôn3ticus
源自 拉丁語 lôn3ticus
from lôna [moon] * see leuk-
源自 lôna [月球] *參見 leuk-

【引伸】
lu.natic
n.（名詞）