# What is a Counterfeiting Word™?

This puzzle is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.

If a word conforms to a special rule I call it a Counterfeiting word™. Here are some examples of Counterfeiting word™ and not Counterfeiting word™:

+----------------------+--------------------------+
| Counterfeiting Word™ | not Counterfeiting Word™ |
+----------------------+--------------------------+
| push                 | first                    |
| fast                 | strong                   |
| noisy                | light                    |
| defend               | against                  |
| old                  | floor                    |
| wet                  | wide                     |
| major                | exit                     |
| wrong                | correct                  |
| below                | optimistic               |
| failure              | all                      |
| lose                 | girl                     |
| soft                 | expensive                |
| include              | hot                      |
| senior               | ascent                   |
| permit               | happy                    |
+----------------------+--------------------------+


And a CSV version for copy-paste purpose:

Counterfeiting Word™,not Counterfeiting Word™
push,first
fast,strong
noisy,light
defend,against
old,floor
wet,wide
major,exit
wrong,correct
below,optimistic
failure,all
lose,girl
soft,expensive
include,hot
senior,ascent
permit,happy


You can check whether a word is a Counterfeiting word™ or not without looking at the words in its column. These are not the only Counterfeiting words™: many more exist.

A Counterfeiting Word™ is...

one where the opposite has the same number of letters.
PUSH/PULL;FAST/SLOW;NOISY/QUIET;DEFEND/ATTACK;OLD/NEW;ADULT/CHILD;WET/DRY;MAJOR/MINOR;WRONG/RIGHT;EXPORT/IMPORT;BELOW/ABOVE;FAILURE/SUCCESS;LOSE/GAIN;SOFT/HARD;INCLUDE/EXCLUDE;SENIOR/JUNIOR;PERMIT/FORBID.

There are no opposites to the right-hand words that have the same number of letters; each has a different number of letters as its opposite. (FIRST/LAST; STRONG/WEAK; etc)

• I like the answer, but what about rot13(yvtug/urnil)? There are other reasonable counterexamples, too, like rot13(svefg/svany naq ershfr/npprcg). Is there some concept of a rot13("eryrinag" bccbfvgr) to use here? Feb 17, 2020 at 20:09
• @mbingo Yeah, I'm not entirely happy with this either -- it seems like some of them are ambiguous. But at the same time, it seems right -- maybe there's some other constraint I'm missing?
– Deusovi
Feb 17, 2020 at 20:11
• Yeah—your list is solid, and rot13("pbhagresrvg" = pbhagre-svg, v.r. bccbfvgrf... gung svg) is appropriate. Feb 17, 2020 at 20:15
• That's the right answer indeed. @mbingo you are right about the definition. I was thinking at: rot13(yvtug/qnex), rot13(svefg/ynfg) and rot13(ershfr/nccebir). But also rot13(ybfr/svaq) so yes, some of them are definitely ambiguous Feb 17, 2020 at 20:19