I doubt that there is a rule for making this very informal expression plural. Some dictionaries, other than those you've cited, show the plural to be no-nos or no-no's. See, for example, Merriam-Webster's Online Dictionary and Dictionary.com.
no-no (nn
)n. pl. no-noes Informal 1. Something unacceptable or impermissible: "Even though his company wasn't the one involved in the case, what he did is considered a definite no-no" (Mike Royko).
2. A social blunder; a faux pas.
no-nos - definition of no-nos by the Free Online Dictionary, Thesaurus and Encyclopedia.
I found that the plural form of no-no is no-noes. Question: Why the second no takes the plural form not the first one? Also, I thought it should be nos-nos. Do we have other words that follow the same pattern? What is the rule behind that?
I doubt that there is a rule for making this very informal expression plural. Some dictionaries, other than those you've cited, show the plural to be no-nos or no-no's. See, for example, Merriam-Webster's Online Dictionary and Dictionary.com.
no–no
noun \ˈnō-ˌnō\
plural no–no's or no–nos
Definition of NO-NO
1
: something unacceptable or forbidden
No-no - Definition and More from the Free Merriam-Webster Dictionaryno–no/ˈnoʊˌnoʊ/ noun
plural no–no's or no–nos
[count] informal 1 : something that people are not supposed to do because it is not proper, safe, fashionable, etc.
▪ Forgetting to introduce your guests to one another is a big no-no when hosting a party. ▪ Sharing prescription medication with other people is a definite no-no. ▪ fashion no-no's
Merriam-Webster's Learner's Dictionary
Thank you, Henry, so much. I followed your instruction and checked some dictionaries. My question is: Do we have a rule in English that we always pluralize the second part of compound word such as no-no not the first part?no-no
[noh-noh] Show IPA
noun, plural no-nos, no-no's. Informal. anything that is forbidden or not advisable, as because of being improper or unsafe: If you want to lose weight, rich desserts are a no-no.
No-no | Define No-no at Dictionary.com
Compound words in English are formed of a descriptive element plus a naming element (in that order), for example birdsong. The naming element is "song" and the description is "bird", which gives a meaning of "song of a bird", which is distinct from songbird, which means "bird that makes song". When compounds are pluralised, the naming element is pluralised, e.g. "police station" > "police stations". In French loanwords, such as "Governor-General" or "court martial", the first word is the naming element, so the pluralisations would be "Governors-General" & "courts-martial".
This aside, "no-no" is a colloquial reduplication, and so the spelling will vary (I hold that the correct way to spell it is "no-no's") but in English it will be seen as a compound and so will follow the rules for compounds when making it plural, i.e. the second element receives the plural.
[Not a teacher]