Copied from a forum:
"Because a long time ago we used to have articles before these numbers like 't sestig, 't seventig 't achtig, 't negentig. (yes sestig and seventig was with a "s")
Now because "'t s" and "'t n" is a bit hard to pronounce the 't died away. 't achtig is easy to pronounce (because achtig starts with a vocal).
That became tachtig and it stayed like that."
:) That's funny! This is a very common expression in Brazilian Portuguese, meaning "all or nothing" or better "it's either too little or too much". Was it a coincidence or some smarty pants put it there on purpose? https://en.wiktionary.org/wiki/ou_oito,_ou_oitenta#Portuguese