不良不忘旧年不纯不忘旧约

This is rich in literary depth, emphasizing fidelity and memory. '不良' ('improper'), and '不纯' ('impure') hint at some moral deviation; but both times it ends with 'not forgetting'. '旧年'(Old Years) likely refers back to former ties, experiences; ‘旧约’(Ancient Agreement) traditionally points to covenants and oaths from days past. Overall it suggests maintaining loyalty and recalling the promises even when personal circumstances fall short.