Everyone knows “หนึ่ง” [nèung] means “one.” But many times, we use the term “เดียว” [diaw] instead, to emphasize the meaning. You can think of it as meaning “only one.” In the contexts where we usually emphasize the fact that there is only one of something, we use the word “เดียว” [diaw] instead of “หนึ่ง [nèung].”
For example, we say “คนเดียว” [kon diaw] instead of “หนึ่งคน” [nèung kon] for “alone.” When you are alone, you are not with anyone else. You’re by yourself. So the meaning is always emphasized. Another example, the term “only child” in Thai we say “ลูกคนเดียว” [lûuk kon diaw] to emphasize the fact that one does not have any siblings and they are the only child of their parents.
On the contrary, when we order something at the restaurant, we use “หนึ่ง” [nèung] for “one (order)” because then, we simply want to say “how many” of something we want. For example, “ขอข้าวเหนียวหนึ่งที่” [kǎw kâao niǎw nèung tîi] “One order of sticky rice, please.” In this case, it would be strange to use “เดียว” [diaw]. Unless the waiter says something like “if you order two, you’ll get another one for free.” then, you can reply saying “ไม่เป็นไร ที่เดียวพอ” [Mâi pen rai, tîi diaw paw] meaning “That’s alright. Just one order is enough.”
Now, let’s take a look at the idiom “รักเดียวใจเดียว” [rák diaw jai diaw]. รัก [rák] means “love” and ใจ [jai] means “heart.” Therefore, “รักเดียวใจเดียว” [rák diaw jai diaw] literally means “(only) one love, (only) one heart.” It is a common expression used to describe a person, man or woman, that is committed to the love (girlfriend/boyfriend/wife/husband) that he or she has. This type of people is the opposite of a “womanizer” which in Thai, we say “คนเจ้าชู้ [kon jâo chúu].” They don’t become attracted or get involved with other girls or guys. Once they are in a relationship, their partner is the only person they pay attention to.
If you are one of those people, you can say “ผมเป็นคนรักเดียวใจเดียว” [phǒm pen kon rák diaw jai diaw] if you’re a guy, or ฉันเป็นคนรักเดียวใจเดียว” [chán pen kon rák diaw jai diaw] if you’re a woman, to describe yourself.
The expression can also be used to describe a person who is single, as long as that person would be committed to their partner once they are in a relationship. A person who is “รักเดียวใจเดียว” [rák diaw jai diaw] is often the ideal partner that everyone wishes to have and is the last type of people that would cheat on their partner.
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