"challenging" in the verb sense
1. challenge, dispute, gainsay
take exception to
"She challenged his claims"
2. challenge
issue a challenge to
"Fischer challenged Spassky to a match"
3. challenge
ask for identification
"The illegal immigrant was challenged by the border guard"
4. challenge, take exception
raise a formal objection in a court of law
"challenging" in the adjective sense
1. ambitious, challenging
requiring full use of your abilities or resources
"ambitious schedule"
"performed the most challenging task without a mistake"
2. challenging, thought-provoking
stimulating interest or thought
"a challenging hypothesis"
"a thought-provoking book"
3. challenging, intriguing
disturbingly provocative
"an intriguing smile"
Source: WordNet® (An amazing lexical database of English)
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