찐 (jjin) in front of a word means the real version of it. 억 (eok) in front of a word means the faked, forced version. 찐텐 (jjin-ten) is genuine excitement and 억텐 (eok-ten) is excitement someone is manufacturing.
찐 is 진짜 (jinjja, real) squeezed into one syllable with a tensed consonant. 억 is 억지 (eokji, forced or strained) cut down the same way. They arrived separately and now function as a matched pair.
찐 is sometimes written jin or zzin. It is not the same sound as 진. The ㅉ is tense, and getting that wrong changes the word.
Real examples
찐친 (jjin-chin)
Real friend, from 친구. Not an acquaintance, not someone from a group chat. The one you would call at 3am.
찐맛집 (jjin-matjip)
A restaurant that is actually good, as opposed to one that is popular because of advertising. The prefix exists because 맛집 lost its credibility to paid reviews.
찐텐 (jjin-ten)
From 텐션. Genuine energy or enthusiasm.
이게 찐이지 (ige jjin-iji)
찐 used as a bare noun. “This is the real thing.”
Now the other side.
억텐 (eok-ten)
Forced enthusiasm. The laugh at a joke you did not find funny, the delight at a gift you do not want. Extremely useful and there is no compact English word for it.
억까 (eok-kka)
Baseless criticism. 억지 plus 까다, which in slang means to trash something. Attacking something on grounds you invented. Common in sports and fandom arguments, and the counterpart 실드 (silde, from “shield”) means defending something equally baselessly.
텐션 does not mean tension
This trips up English speakers every single time, and it matters for both 찐텐 and 억텐.
Korean 텐션 (tensyeon) borrowed the English word and kept none of the anxiety. It means energy level, liveliness, how high your mood is running. 텐션이 높다 is a compliment describing someone lively. It has nothing to do with stress or conflict.
So 억텐 is not forced tension. It is performed high spirits, the social energy you produce when the room expects it and you have none.
억 is also the number
Separate word, same syllable, and this one causes real confusion.
억 (eok) means one hundred million. 억대 연봉 (eokdae yeonbong) is a salary in the hundreds of millions of won. 십억 (sibeok) is a billion.
There is no relationship between the number and the prefix. The number stands alone or attaches to counters, and the prefix is welded to the front of a word describing behavior. If you see 억 next to a number or a currency, it is money, and if you see it glued to 텐 or 까, it is the prefix.
Two things to be careful with
찐 is not a general intensifier. It means authentic, not very. 찐 좋다 is not how you say something is very good. For that you want 완전 (wanjeon), 진짜 (jinjja), or 너무 (neomu), all of which are neutral and safe in far more situations than 찐 is.
And not every word starting with 찐 uses this prefix. At least one common insult happens to begin with the same syllable and has nothing to do with 진짜. If you see 찐 attached to something you do not recognize as a normal noun, do not guess and do not repeat it.
Where they do not go
Neither belongs in anything written. Both are two-letter clippings of longer words, so the effect in a formal register is roughly like abbreviating in a cover letter.
억까 has an extra problem, which is that it is an accusation. Saying someone is 억까하는 중 means their criticism is dishonest, not merely wrong. In a workplace or a serious discussion that is a much bigger claim than the casual tone of the word suggests, and it tends to end conversations rather than continue them.
찐친 is the one exception in tone. It is warm, and among friends of similar age it is completely ordinary in speech, though still not in writing.
I would like to know what other languages do with 억텐, because English seems to need a whole sentence for it. Japanese has 空気を読む for the adjacent idea of reading the room, but that is the skill rather than the performance. If your language has a single word for visible fake enthusiasm that everyone present can see through and nobody mentions, I would be curious what it is.