The spelling of the word "nally" may seem simple, but the pronunciation is quite different than expected. In fact, the IPA phonetic transcription for "nally" is /ˈnæli/. This means that the first syllable is pronounced with a short "a" sound, followed by a long "e" sound in the second syllable. The spelling of this word can be confusing because the "a" and "e" sounds are typically not paired together in English. However, once the pronunciation is understood, the spelling becomes easier to remember.
The word "nally" is not listed in any reputable dictionaries nor does it have an established etymology. It appears to be a rare or possibly informal word, so its origin and meaning may be unclear.