Indicates that something happens with some regularity, but not always. Often used in formal or written contexts. Can imply 'more often than not' or 'it is not uncommon that...'.
Also means 'often' or 'frequently', but 往々 carries a nuance of 'it tends to happen' or 'as is often the case', and is slightly more literary.
Etymology
From the reduplication of the kanji 往 (going, past), literally 'going and going', which evolved to mean 'sometimes' or 'often' in the sense of recurring events.
