Far-right extremists use the internet to recruit and connect with other radicalised individuals and spread their propaganda online, with them being prolific users of online forums and social media. Such online data can be analysed using linguistic tools to identify markers of radicalisation in social media, forum posts, and other extremist texts. Few studies have examined how language and sentiment online evolves in extremist forums, which provides insight into how radicalisation into extremism unfolds overtime. To address this gap, we examine how the extremist language used by Australian members of the online far-right forum Iron March changed in relation to the level of engagement on the forum, as well as assessing whether any relationship existed between online extremist sentiment and offline violence. Forum data were computationally coded and analysed using the Grievance Dictionary, which is designed to identify psycholinguistic indicators of extremism. Results indicate that while extremist language changes over time, there was no support for the suggestion that greater forum engagement results in higher levels of extremist sentiment. The results found that a high level of online extremist sentiment is associated with offline violence. Findings are linked to existing literature, and limitations in the study design are acknowledged.