Per Wikipedia: https://en.wikipedia.org/wiki/Spanish_profanity#Pendejo
Pendejo (according to the Diccionario de la lengua española de la Real Academia Española, lit. 'a pubic hair'), according to the Chicano poet José Antonio Burciaga, "basically describes someone who is stupid or does something stupid." Burciaga said that the word is often used while not in polite conversation. It may be translated as "dumbass" or "asshole" in many situations, though it carries an extra implication of willful incompetence or innocent gullibility that is ripe for others to exploit. The less extreme meaning, which is used in most Spanish-speaking countries, translates more or less as "jackass". The term, however, has highly offensive connotations in Puerto Rico. An older usage was in reference to a man who is in denial about being cheated (for example, by his wife). Burciaga said that pendejo "is probably the least offensive" of the various Spanish profanity words beginning with "p" but that calling someone a pendejo is "stronger" than calling someone estúpido. Burciaga said, "Among friends it can be taken lightly, but for others it is better to be angry enough to back it up."
In Mexico, pendejo most commonly refers to a "fool", "idiot", or "asshole". In Mexico, there are many proverbs that refer to pendejos. In Peru, it means a person who is opportunistic in an immoral or deceptively persuasive manner (usually involving sexual gain and promiscuity but not limited to it), and if used referring to a female (ella es pendeja), it means she is promiscuous (or perhaps a swindler). There the word pendejada and a whole family of related words have meanings that stem from these. In South America, pendejo is also a vulgar, yet inoffensive, word for children. It also signifies a person with a disorderly or irregular life. In Argentina, pendejo (or pendeja for females) is a pejorative way of saying pibe. The word, in Chile, Colombia, and El Salvador, can refer to a cocaine dealer, or it can refer to a "fool".
In Puerto Rico and the Dominican Republic, it has different meanings depending on the situation. It can range from ¡Te cogieron de pendejo! ("You were swindled!") to ¡Qué tipa pendeja! ("What a dumbass!" as when a strange woman behaves offensively and then suddenly leaves). In Mexico and some countries in Central America, especially El Salvador, una pendejada/pendeja is used to describe something incredibly stupid that someone has done. In many regions, especially in Cuba, pendejo also means "coward" (with a stronger connotation), as in ¡No huyas, pendejo! ("Don't run away, chicken-shit!") or No seas pendejo! ("Don't be such a coward!").
In South America, it refers to a person regarded with an obnoxiously determined advancement of one's own personality, wishes, or views (a "smartass"). In Argentina, Chile, and Uruguay, pendejo or pendeja refers to a child, usually with a negative connotation, like that of immaturity or a "brat". Also in Argentina, since pendejo literally means "pubic hair", it usually refers to someone of little to no social value. In Peru, however, it does not necessarily have a negative connotation and can just refer to someone who is clever and street-smart. In the Philippines, it is usually used to refer to a man whose wife or partner is cheating on him (i.e. a cuckold). In North Sulawesi, Indonesia, pendo (a derivative of pendejo) is used as profanity but with the majority of the population not knowing its meaning. The word was adopted during the colonial era when Spanish and Portuguese merchants sailed to this northern tip of Indonesia for spices. In the American film Idiocracy, Joe Bauers's idiot lawyer is named Frito Pendejo. Burciaga says that the Yiddish word putz "means the same thing" as pendejo.
In Russian we have a common slur that covers a lot of cases like pendejo, but sadly has a heavy luggage of homophobia. I'm surprised and delighted pendejo writeup don't have any of it.
