He compiled a Malayalam-Portuguese dictionary, known as "A Breve Instrucção de Lingoa Malaiala", which was published in 1686. This dictionary was a valuable resource for early European scholars and missionaries who were learning Malayalam.
However, it's important to note that there were earlier attempts at documenting Malayalam vocabulary. For example, The "A Breve Instrucção de Lingoa Malaiala" was based on the work of Fr. Vincenzo de Sanctis, who had prepared a lexicon of Malayalam words in the 16th century.
Therefore, while Arnos Pathiri's dictionary was the first published, it built on the work of earlier scholars.