Shall I use 'thus' or 'thusly'? [duplicate]

The suffix -ly is used to form adverbs out of words that aren't already adverbs. "Thus" is already an adverb. There is no word "thusly." "Thusly" is sometimes used as a joke to indicate that the speaker is pretending to adopt a fancy style of speech and using a word that doesn't exist. A similar joke-word is "bestest;" "best" is already the superlative, so it doesn't need another "-est."

The American Heritage Dictionary of the English Language says:

Thusly is occasionally employed humorously, for mock-stylish effects. Otherwise, as a variant of thus (itself an adverb), thusly is termed unacceptable by 97 per cent of the Usage Panel.