Recently, music complexity has drawn attention from researchers in Music Digital Libraries area. In particular, computational methods to measure music complexity have been studied to provide better music services in large-scale music digital libraries. However, the majority of music complexity research has focused on audio-related facets of music, while song lyrics have been rarely considered. Based on the observation that most popular songs contain lyrics, whose different levels of complexity contribute to the overall music complexity, this paper investigates song lyric complexity and how it might be measured computationally. In particular, this paper examines the concreteness of song lyrics using trend analysis. Our analysis on the popular songs indicates that concreteness of popular song lyrics fell from the middle of the 1960s until the 1990s and rose after that. The advent of Hip-Hop/Rap and the number of words in song lyrics are highly correlated with the rise in concreteness after the early 1990s.