[944] Loving-kindness (mettā), in the phrase "One should suffuse with loving-kindness beings both trembling and firm (mettāya phasse tasathāvarāni)," is that which, toward beings, is friendliness, the act of being friendly, the state of friendliness; kindliness (anudā), the act of kindliness, the state of kindliness; the seeking of welfare, sympathy, non-ill will, non-malevolence (abyāpajjha), non-hatred (adosa), a wholesome root.
"Trembling (tasā)": those by whom the craving (taṇhā) that causes trembling (tasiṇā taṇhā) has not been abandoned, and by whom fears and terrors have not been abandoned. For what reason are they called "trembling"? They tremble, they are alarmed, they are terrified, they fear, they fall into terror; for that reason they are called "trembling."
"Firm (thāvarā)": those by whom the craving that causes trembling has been abandoned, and by whom fears and terrors have been abandoned. For what reason are they called "firm"? They do not tremble, they are not alarmed, they are not terrified, they do not fear, they do not fall into terror; for that reason they are called "firm."
"One should suffuse with loving-kindness beings both trembling and firm": one should touch, should pervade, both the trembling and the firm with loving-kindness; one should dwell having pervaded them with a mind (citta) accompanied by loving-kindness — abundant, exalted, immeasurable, without enmity, without malevolence. Hence, "One should suffuse with loving-kindness beings both trembling and firm."
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.