1. (adverb) through chance,

• perchance

2. (adverb) by accident

• by luck
• haply

3. (adverb) without advance planning

• accidentally
• circumstantially
• unexpectedly

1. (adverb) without advance planning

• advisedly
• by choice
• by design
• deliberately
• designedly
• intentionally
• on purpose
• purposely

1. (adverb) without advance planning

• unexpected