Definition of "plangent"

[S]how him a refined or powerful face, let him hear a plangent or a penetrating voice […] and his mind was instantaneously awakened.

1879, Robert Louis Stevenson, “chapter 1”, in The Story of a Lie

Since mid-day their plangent, disquieting cries had foretold its approach.

1919, Ronald Firbank, Valmouth (Duckworth hardback), page 49

In the lament about the massacre — the work’s second movement — he entered a more urgent register in the high reaches of the cello, but the sense of grief was more plangent than raw, devoid of any real outrage.

2013 Sept. 22, Corinna da Fonseca-Wollheim, “Music Review: A Middle East Mourned and Celebrated in Suites”, in New York Times‎

What central sea with plume-plucked midnight strewn,Plangent to what enormous pleniluneThat lifts in silence, hinderless and stark?

1922, Clark Ashton Smith, Desire of Vastness