劣 is rarely used alone — it commonly appears in compounds: 低劣 (low and inferior), 恶劣 (bad, vile, severe), 劣质 (of poor quality), 劣势 (disadvantageous position). The opposite is 优 (superior, excellent).
Top 少 — "few, lacking" — combines with strength below to give the literal idea "little strength," from which came the metaphorical sense "inferior, of poor quality." Pure compound-ideograph logic: not a phonetic, purely meaning-driven.
