This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

ad. Without making a mistake, perfectly, directly

ad. In an unerring manner.

In an unerring manner; without error, mistake, or failure; infallibly.

ad. without making errors

"He picked the name unerringly out of the hundreds he carried in his mind, and along with it came automatically all he knew of young Heinrich's history, family, and connections, these appearing in his imagination round the boy's long, dreamy face in a constellation of social affinities, orderly and complex as the pattern of a snowflake."